As a matter of fact, I once heard Fiddlers green sung at a GAA SCOR Competition, The competitor was not dsqualified even though all the songs were supposed to be irish, in this case it was probably just ignoramce of its origins.
on another occasion i was judging a singing competition and the performer sang caledonia a song written by dougie maclean and as i understand it a scottish song, again the performer was ignorant of the songs origins, again this was supposed to be songs that were irish ,it was supposed to be a competition promoting irish culture. so i have first hand experience that confirms what jack is saying, that songs are often assumed to be irish,even in competitions that are supposed to be exclusively about irish musical culture.
I have also occasionally encountered astonishment on the part of irish people that the english have any trad songs or tunes, I have also met irish people who are very well acquainted with many english trad songs.