The appellation “Celts” crystallized out of a variety of references by classical (Greek or Roman) authors identifying tribes that were neither Scythic nor Germanic. The languages now known as “Celtic” consist of two clusters (Welsh/Breton and Cornish; Irish/Scottish Gaelic and Manx) which are mutually unintelligible. Since 17th- and 18th-century antiquarians considered Welsh to be a descendant of the language of the ancient Britons, and Breton of ancient Gaulish (the former supposition being by and large correct, the latter erroneous), the term “Celtic” was applied to all of them in their family relationship once the relatedness of Welsh/Breton and Gaelic was demonstrated (c.1700). How this Celtic language family fitted into the larger scheme of linguistic relations remained unclear until the emergence of the Indo-European paradigm around 1800, and indeed for some decades it was doubtful whether the Celtic languages could be classified as Indo-European at all. That matter was settled by a series of studies initiated around 1820 by Adolphe Pictet and Franz Bopp, and culminating in J.C. Zeuss’s Grammatica celtica of 1853.

Around the time of Zeuss’s comparative Celtic grammar, the excavations of the Halstatt necropolis near Salzburg (1847-63) and of the La Tène settlement near Neuchâtel (1857-63) tied these linguistic categories in with material remains. By the mid-19th-century, the term “Celtic” was consolidated and gained wide currency to refer to the collectivity of the Iberian, Gaulish, Helvetic, British and Gaelic tribal cultures of antiquity, and to a set of languages spoken on the western fringes of the British Isles and in Brittany.

By this time “Celtic” also came to stand for a cultural tradition, manifested originally in the (spurious but appealing and memorable) figurehead of the putative 4th-century Scottish bard Ossian. Ossian, purportedly discovered, and edited and translated by (but in fact largely the fanciful projection of) the Scottish antiquary James Macpherson in the 1760s, was endowed with qualities that were becoming fashionable at that time: sublime in sentiment, nature and rhetorical style; an “original primitive genius” like Homer, creating not from acquired skill but from spontaneous, superhuman inspiration; yet unlike Homer in that his effusions are more sombre and meditative. If the sunny, Mediterranean Homer was the presiding genius of Europe’s classical tradition and sanguine Enlightenment vitalism, Ossian became his northern, more melancholic counterpart: the prototype of the Counter-Enlightenment and of Romanticism. This opposition, schematized by Goethe in his Die Leiden des jungen Werthers of 1774 and by Mme de Staël in her De la littérature of 1800, made Ossian (even after Macpherson’s pretensions to his historical authenticity had been exploded in an epoch-making scandal) the symbolic figurehead of European Romanticism. By the same token, the cultural temperament that was ascribed to the newly-identified “Celtic” nations was Romantic and Ossianic: the Irish, the Welsh, the Bretons and the Scottish Highlanders were seen (with some debt to Caesar’s description of the Gauls) as the opposite of rational. (This in fact resembled the equally emotional and Romantic temperament that was ascribed to the “Slavs”, who as a cultural category emerged in these same decades.) Celts were considered emotionally volatile, given to fancy and speculation, dreamers in – as the catchphrase had it – “stubborn rebellion against the despotism of fact”.  This was also seen as the underlying reason why their culture had been in constant retreat for the past 1000 years; in Ossianic style, the Celts “went forth to battle, but always fell”; and the fact that they were now seen as clinging to the last remaining peripheral footholds on the Atlantic margins of north-western Europe gave them the Romantic quality of being the last remnant of a tradition in statu moriendi. Overtaken, like other noble savages such as the Mohicans, by the superior pragmatism of modern France and Britain, they were yet capable of inspiring a wistful nostalgia for a life beyond mere pragmatism.

This fey, ultimately Ossianic ethnotype was habitually applied to the Bretons and their doomed resistance against the advent of modernity in post-1789 France; to the Scottish Highlanders evoked by Sir Walter Scott as a dwindling tradition broken in the doomed 1745 rebellion; and to the Irish and Welsh as well. At the same time, the typically Romantic interest in salvaging the last surviving tenuous remains of a receding past created much Celticist goodwill: Scott enshrined Highland dress and traditions into the iconography of modern Scotland even as the Highland Clearances were taking place, and tales set in the dwindling outposts of Celtic culture (the picturesque landscapes of Wales, Brittany, Ireland) were as appealing to European readers (Souvestre, Lady Morgan) as the Mohican tales of Fenimore Cooper were to Americans.

The common appellation and self-identification as “Celtic” was first used for consciousness-raising purposes in Brittany. As the theory deriving the Bretons from the ancient Gauls lost ground, modern Bretons sought to align themselves instead with their counterparts across the sea, in Wales. The Welsh-Breton axis was first established in the contacts between Thomas Price and Le Gonidec, giving the latter a commission to translate the Bible under the auspices of the British and Foreign Bible Society, and support for his lexicographical work, and inspiring in the former the theory of a Welsh-Breton conduit spreading the Welsh-British literary legends of King Arthur (the matière de Bretagne) to medieval France and thence to Europe. From the late 1820s on, prominent Bretons (La Villemarqué among them) were fêted guests at Price’s Abergavenny eisteddfod; later that century, they were to carry the format of the eisteddfod and the gorsedd (a quasi bardic-druidical assembly fancifully created by the fantastical Welsh antiquary Iolo Morganwg) back to Britanny. It was this Breton desire to hook up with the more robust Welsh cultural institutions that began to give the idea of “Celtic” a more programmatic, mobilizing charge. “Celtic” banquets were held, and (in obvious analogy to the Pan-Slavic congresses of 1848 and 1862) Pan-Celtic congresses were planned. A “Breton eisteddfod”, planned in Quimper for 1866, was banned by the French government; but Fêtes celtiques were held in 1867 in Saint-Brieuc, in 1869 in Morlaix and again in Saint-Brieuc in 1906. The 1867 event led to the establishment of the Revue celtique in 1873.

Chairs of “Celtic” were established around the same time: in 1869 in Oxford, in 1882 at the Collège de France. The first incumbent of the French chair, Henri d’Arbois de Jubainville, brought out his authoritative Introduction à l’étude de la littérature celtique in 1882. In Germany, the heartland of comparative philology in those decades, the study of Celtic was academically institutionalized with the founding of a chair in Leipzig in 1899, followed by the establishment of the Zeitschrift für Celtische Philologie in 1901. This prestigious academic interest was a great symbolic support for the peripheral communities where a Celtic language was spoken or a “Celtic” past was remembered.

Meanwhile, the Breton-born Ernest Renan had mapped the post-Ossianic, Romantic temperament of “the Celt” onto literary studies with his essay La littérature des races celtiques (1854). It nostalgically opposed the ethnotype of the dreamy, otherworldly Celt to the soulless pragmatism imputed to modern France and Germany. The ethnotype was taken over by Matthew Arnold, who, inspired by a visit to an eisteddfod, and mindful of his mother’s Cornish ancestry, amalgamated the ideas of Renan, Zeuss and philologists like O’Donovan, O’Curry and Stephens into his Oxford lecture series On the study of Celtic literature (published in 1867). Arnold (like Renan, but with more recourse to recent philological developments) vindicated the Celtic literary tradition as an important part of British culture, leavening “Saxon” stolidity and pragmatism with the fancy and Romantic dreaminess of Wales and Ireland. This ethnotype, now given fresh cultural prestige, was eagerly embraced by the fin-de-siècle poets of the Irish Literary Revival, who, while writing in English, felt they were expressing a Celtic temperament in their aversion to Realism, Naturalism and everything English, including the modernizing process as such. The Irish Literary Revival in turn did much to provide an appealing cultural ambience for separatist Irish nationalism.

In the final decade of the 19th century, Welsh-Breton Celticism meshed with the self-defined “Celts” of Ireland and Scotland. Cultural festivals in the mode of Gaelic revivalism had already been established on the model of the Welsh eisteddfod. In 1892 the first Scottish Mòd was organized by An Comunn Gaidhealach; in 1897 the first Oireachtas was organized by its Irish counterpart, the Gaelic League. Shortly thereafter, Patrick Pearse, as a delegate of the Gaelic League, attended a Welsh eisteddfod (1899). Pan-Celticism had meanwhile become a presence on the Dublin cultural scene: a Pan-Celtic Society, inspired by Breton activist ideas, had been founded in 1888, and in 1889 it had published a volume of Lays and lyrics of the Pan-Celtic Society, followed, in 1900, by a periodical Celtia. These initiatives had met with scepticism from the more activist members of the Gaelic League, who preferred to focus their cultural revival on their own nation rather than dispersing their energies in a wider ambit; but after Pearse’s eisteddfod visit of 1899 the Gaelic revivalists embraced the Celtic identity and their Celtic brethren, and a Pan-Celtic Congress was held, chaired by Douglas Hyde, in Dublin in 1901. A second congress took place in Caernarfon in 1904; a third in (again) Dublin in 1907. The 1917 Celtic Congress in Birkenhead consolidated the event, which has become a regularly recurring occasion.

The self-identification of a peripheral minority culture as “Celtic” had meanwhile spread, implausibly, to Spanish Galicia, where the historian Manuel Murguía in the 1880s had appropriated the Irish/Gaelic origin myth (as related by D’Arbois de Jubainville’s Introduction à l’étude de la littérature celtique of 1882) that the Gaels had settled Ireland from a departure point in north-western Spain. Galicia was thus, by Murguía and others in the Galician rexurdimento, glamourized as the pre-Irish place of settlement of the wandering Gaelic tribes, and, hence, as a part of Celtic Europe. Galicians after World War II tried to affiliate themselves to the Celtic League (founded in 1961 as a political outgrowth of the Celtic Congresses) and the Festival Inter-Celtique, held annually in Lorient since 1971.

Except among a small group of fringe activists around the periodical Carn (founded in 1973), Pan-Celticism never became a serious political nationalist movement; a union of independent Celtic nations was never a viable agenda. However, as a thought experiment among those identifying with a “stubborn rebellion against the despotism of fact”, a sense of Celtic solidarity has done much to bolster initiatives in political and cultural nationalism in Brittany, Wales, Ireland and, to a lesser extent, in Scotland. In addition, Pan-Celticism, as a movement and as a sense of collective identity, shored up the attempts to revive extinct Celtic languages like Manx and, especially, Cornish. The Cornish language had died out in the late 19th century, but Cornish regionalism caught the tide of Pan-Celticism in the late 19th century and, with activists like Henry Jenner (1838–1934) and Henry Morton Nance (1873–1959), and sympathizers like the esteemed critic and Cambridge professor Arthur Quiller-Couch, launched a moderately successful language-revivalist and regionalist movement. Cornish Celticism and Arthurianism also inspired an undercurrent of Celticism within English culture.

As with other macronationalistic “pan-movements”, Celticism, while never a politically mobilizing programme, did provide the cultural-nationalist movements in the communities concerned with an amplifying “sounding board” effect. For Irish, Welsh and Breton cultural practitioners, the invocation and even the nomenclature of the larger ethnic-philological category bestowed prestige  and obviated political contentiousness. Thus, the earliest efforts to have Gaelic placed as a subject on the Irish teaching curriculum succeeded because the language was introduced under the label “Celtic”.