If you look at the repertoires of Nic Jones and Andy Irvine you'll see quite a few songs that they had in common - Barrack/Patrick Street, Courtin' is a Pleasure/Farewell Ballymoney, The Green Mossy Banks of the Lee, etc.

However, I don't think that one stole from the other. Rather, they both have very similar tastes and probably arrived at those songs independently of each other. Both of them also have the habit of altering melodies and words to mould the songs to their needs.