By the 1890s, the notion of Celticism was associated, within English culture, with the dreamy otherworldliness of romance and the supernatural. That ethnotypical connotation had been introduced by Matthew Arnold, who in his On the study of Celtic literature (1867) had argued that the difference between the English and the German temperaments was that the “Saxon” sense of pragmatism common to both was leavened, in the English case, by an admixture of Celtic fancy and otherworldliness. As a result, a taste for fairies and romance (Arthurian or Mabinogion-inspired) was often characterized as “Celtic”. Fairy folklore and romance, both literary and scholarly, gravitated to Celtic titles: Joseph Jacobs’s Celtic fairy tales (1892), William and Elizabeth Sharp’s collection Lyra Celtica: An anthology of representative Celtic poetry (1896), John Rhŷs’s Celtic folklore: Welsh and Manx (1901) and W.B. Yeats’s Celtic twilight: Faerie and folklore (1902). Hence, a “Celtic” aesthetic programme like that of Yeats could easily fit a larger British ethnocultural frame; as such it was mistrusted by Irish cultural nationalists, who preferred to label their country’s native culture Gaelic or Irish rather than the English-anchored exonym “Celtic”.

Conversely, a sense of Celticity was embraced by those British cultural producers who wanted an alternative both to the Nordic-Germanic repertoire of the previous, Saxonist generation and to French decadence. In British musical life the main representatives were Arnold Bax and Rutland Boughton. Bax (1883–1953) in the early years of the century drew on Irish materials, thus in the tone-poems Into the twilight (1908), In the Faëry hills (1909) and The garden of Fand (1913). Shocked by the Easter Rising, he moved away from anything Irish; his last Celtic tone-poem was Tintagel (1917), evoking the fabled Cornish castle with its Arthurian associations.

Boughton (1878–1960), whose composing technique was inspired by Wagner but whose imagination was Arthurian, composed a Ring-style tetralogy of operas on Arthurian themes: The birth of Arthur (1909), The Round Table (1915-16), The Lily Maid (1933-34), Galahad (1943-44) and Avalon (1944-45) – the last two have remained unperformed, indicating the outmodedness of the format by the 1940s. Boughton sought to advance his programme by establishing a Bayreuth-inspired festival at Glastonbury (a place with strong Arthurian links) in 1914; it ran with considerable success until 1926. Boughton also used other “Celtic” themes for his compositions: The queen of Cornwall (1924; based on a 1923 play by Thomas Hardy reworking the Tristan and Isolde theme) and, most successfully, The immortal hour (1912-13), based on a play by Fiona Macleod published in 1908. This “Fiona Macleod” was in fact none other than the aforementioned William Sharp. Almost symbolically for the dualism of English aestheticism and Celticism (and its genderization), Sharp (1855–1905) wrote in two personalities; when writing in the “Celtic” fairy-aesthetic mode, he channelled the persona of an inner Scotswoman, “Fiona Macleod”.