They disappeard completely with The Public Dance Halls Act - 1935, when the government introduced a tax on all such gatherings

That's interesting. In 1943 de Valera gave a St Patrick's Day speech about his ideal Ireland, which is widely remembered as including a line about comely maidens dancing at the crossroads. Actually there's no reference in the speech to dancing, at the crossroads or anywhere else. If they died out in the mid-1930s, it's easy to see why the image of dancing at the crossroads would have attached itself to the speech in people's minds. By 1943 it would have been a powerful image of bygone rural Ireland - definitely a thing of the past, but still within living memory.

(Also, if de Valera's own government effectively banned dancing at the crossroads, it's not hard to see why he wouldn't have mentioned it!