This is from the Roche collection and I pass it on with the thinking that you can never have too many good hornpipes!
This is my arrangement which includes a few small changes - the original can be found on my webABC site at www.capeirish.com/webabc/collections/roche/roche-tunes.html#1981.
Roche gives the Gaelic title as "Ingar Do Shuidhe Finn" which translates to "Perpendicular You’re Sitting Fair One" (at least according to my Dineen’s) - I have the feeling I’m probably missing something, so I’ll leave a sensible translation to anyone more fluent in Irish than I am. I’m not sure where the title "Butter Boys" comes from.
This is one of those tunes where it appears that the A and B parts are reversed.