An interesting early 20th century photograph of people out dancing in London (click on photo to enlarge). It comes from Living London by GR Sims, published in 1902, with the caption 'Learning Irish reels (Athenaeum Hall, Tottenham Court Rd.)' The dances were organised by the Gaelic League, which promoted Irish culture. The Athenaeum Hall incidentally was used for lots of radical meetings - William Morris was among those who spoke there.
From Moving Here: 'Living London records that the Gaelic League 'holds meetings for practice every Monday evening, when jigs, three-part and four-part reels, "heel and toe", "cover the buckle", and other complicated steps are taught to novices or practiced by experts'. As this photograph reveals, traditional Irish dancing around 1900 did not necessarily involve the wearing of 'traditional' Irish costume. This evolved during the 20th century as a colourful and distinctive Irish dancing dress. It is said that a group of London-based Irish, visiting Macroom feis (festival) in August 1900 were the first to wear the kilt - an interesting example of how malleable Irish culture could be. Dance halls in towns and cities in England were an important social venue where Irish people could meet'.