I didn't attend on 14 June, but if "Buachaillin dhoun" was indeed sung in Irish, than the title should be spelled "An Buachaillín Donn" (the brown-haired lad) - "Dhoun" is an Anglicised spelling. The song also known as "Mo Bhuachaillín Donn", my brown-haired boy, which is sung mainly in English. This site mistakenly translates the title as "the fair-haired boy". Donn means brown; fair haired would be "bán" or "fionn".
https://www.irish-folk-songs.com/mo-bhuachailliacuten-donn-lyrics-and-chords.html

same song https://comhaltas.ie/music/detail/an_buachaillin_donn

Mudcat has the same song posted as "BOUCHALEEN DHOUN" but that is not Irish spelling; it is a rough transliteration.

I've now convinced myself that it was this song which was sung at the mudsing in question, a song in English with a few words in Irish Gaelic included.

[I have edited the song list in accord with this post. Thanks! Gerry]