250 of the 842 tunes in P. W. Joyce's 'Old Irish Folk Music and Songs' (1909) are at faeryland.tamu-commerce.edu/joyce
At the end of the page (which takes a while to load) is a link to the online O'Neill project, but it wouldn't work for me.

Mel Bay now distributes the former Collins reprint of O'Neill's 'Music of Irleand' with a list price of $25, but I couldn't find it listed on the Mel Bay website.