I think it may be in one of Alison Kinnaird's collections. It's known only from Scottish sources, and I've never heard anybody seriously suggest it was Irish (at the other extreme, one author says it is the *only* known early Scottish harp tune). You certainly don't find stuff like that in O'Neill's 1001. Rory Dall's identity is mysterious; there may have been two of him, one Irish and one Scottish, 100 years apart.

It's also known as "Lament for the Harp Tree" - probably a mistake, but people have come up wth the idea that the harp was a "tree of strings" to explain it, hence the title of Kinnaird's major book.