I've been collecting material from whereever I could get it since 1965. I retired late in 1988 and started going to the Library of Congress and Folger Shakespeare Library early the next year. I xeroxed Lyle's version of "Pretty Peggy of Derby, O" from the Folger Library copy of Lyle's book, and found the old chapbook with the original version in the Library of Congress. Also in LC is the pirated musical score for 'The Foundling of the Forest', with the tune for "The Landlady of France", mis-attributed to Michael Kelly, and Samuel Arnold's score for 'Two To One', 1784, with the earliest copy of the tune. The Library of Congress is lacking the 4th part of O'Farrell's 'Pocket Companion for the Irish or Union Pipes', so I didn't get the "Dandy O" version of the tune until recently (although I had figured out that's where O'Neill got his copy for his Irish Dance Music, since the index is in the third part). LC also has Brysson's collection of 1790 with the tune. I forgot how I found out the tune was in the 3rd volume of Aird's Airs, but the National Union Catalog showed one copy of the book in the USA. I wrote to the library that had it, requesting a copy, and got it. The book has subsequently been sold, and I don't know where it is now.

You look where you can and keep notes on just about everything you find, and it may take years, like on "Pretty Peggy of Derby, O" and "Druimion Dubh" before you have enough to start putting the pieces together and start looking for specifics to fill in the gaps on single songs or tunes.