Probably the best explanation of why so many of the older songs , written by English people, were taken on by Irish singers is, imitation.
The Music Hall performers in the Irish cities, and touring companies in the town halls, brought British acts to Ireland, singing THEIR popular British songs. The fact of the British Army being stationed here, in Barracks spread Nationwide and especially in Dublin, up to the 1920s, is another.
These people came from all over Great Britain and whether as Theatre acts or as soldiers, singing for their own entertainment on their nights out in the pubs, would have inspired local people to imitate them and copy their songs , with, perhaps sometimes, changes, to render them more palatable to Irish tastes. Composition of original songs in their style would, inevitably, have followed.
This is a trait that we Irish have perfected. Over the years, we have had to learn to adopt and adapt, in order to survive, so don't be surprised that Christy Moore or Luke Kelly or whoever, did just that when performing in British Folk Clubs.
Because we were denied the performance of our own cultural styles, in public, for generations , may also have had something to do with the necessity to make those adjustments.
Just saying..