The Session <http://thesession.org/> is a "jam session" resource for primarily Irish trad. musicians. It contains a a listing of sessions (e.g., sessions in Barcelona), a discussion forum, etc. Most importantly for linking to from MusicBrainz would be their repertoire of traditional tunes with ABC notation and in some cases scores/sheetmusic as GIF files, as well as their directory of recordings which feature these tunes.

Here's what The Session says:

The exchange of tunes is what keeps traditional Irish music alive. This website is one way of passing on jigs, reels and other dance tunes.

Some of the tunes are well known, and some are more obscure. It's this mixture of the familiar and the new that makes for a good session.

Their "tunes" would be linked with Works and their "recordings" would be linked with Release Groups.

Far most of the tunes listed are traditional (ie., public domain) and the few that aren't strictly traditional, are traditional enough to be unlikely to cause any legal issues.

The site contains a lot of information about tunes that are the same, but use different names, as well as which tunes are played on what recordings/Releases, and the discussion/comments for the individual tunes also occasionally contain valuable information about the tune's legacy.

Examples:
http://www.thesession.org/recordings/display/4260
http://www.thesession.org/tunes/display/388

Discussed briefly on IRC:
http://chatlogs.musicbrainz.org/musicbrainz/2012/2012-10/2012-10-31.html#T20-16-03-958160

Chat about RFC process:
http://chatlogs.musicbrainz.org/musicbrainz/2012/2012-11/2012-11-05.html#T17-12-37-624684