The lyrics and background notes to Úna Bhán as provided above by John are also at ingeb.org
That site also includes verses in poetic German translation by Heinrich Möller and a midi/sound file. The oldest known transcription of the song had 45 verses so it's no wonder that different singers have a different selection.

Máire Áine Ní Dhonnachadha's version with translation and notes are at Áine Cooke's webpages Máire Áine recorded Úna Bhán both on Claddagh records (Ireland) and on folkways (USA)

A shorter version and abc are at Liam Hart's archives

For sheet music you have to go off line for publications such as 'Cuisle Cheoil' (Dublin:an Roinn Oideachais [Dept of Education],1976); Fleur Roberts, ed. 'Irish Ballads',Dublin: Gill and MacMillan, 1996); Seán Óg Ó Baoill and Manus Ó Baoill, 'Ceolta Gael', Cork: Mercier, first ed. 1975.