Whatever one thinks about the status of the six counties of Northern Ireland the one thing it isn't is a single nation - so a national anthem can't exist for it. It would be nice if there could be a song that everyone could unite behind, but I think it will take time and hard work before that could be achieved. Danny Boy is useful suggestion, but unfortunately once you use the alternative title the row over "London"Derry starts off. And the words of the song, while fine (if a bit mawkish)in that context hardly form the basis of an anthem.
There was a new set of words written for this song (I think by Tyrone Guthrie but I might be completely wrong about this) and recorded by De Dannan some years back. These words referred to 'a land by love united' which is at least a lovely aspiration, but a while off I think. The song also contains at least one reference to God which will probably upset the atheists (both the catholic atheists and the protestant ones).
One thing that gives me hope is when I see the South African Rugby team playing and the (still mostly white) crowd passionately singing the old ANC anthem. Who'd have thought that 20 years ago ?