Ahh go on! Give me a clue as to where this pub is. Your best bet is to get one of the many compilation songbooks "Best of Irish Ballads". They will list songs considered to be Trad or Public Domain, which includes compositions by Ewan MacColl, Ralph McTell and many others. Don't forget "Fiddler's Green", John Conolly has long ago given up the battle, or Belfast Mill ( Si Kahn was almost physically attacked for suggesting it was based on his song Aragon Mill).These are now trad. in some good songbooks. I think the spies and informers are only concerned with pop music. As regards copywrite on arrangements, learn the song from the original source singer, which is often much better than fancy arranagements anyway. If you play guitar, leave it slightly out of tune, that way they have no comeback on you.
Meanwhile, enjoy the Guinness. If you can't beat them, join them!