So sad to see this beautiful song yet again marred by misguided patriotic bigotry. It was written by an ex-patriate Scot, who has spent most of his life in Australia, and it could just as well apply to a soldier from any of the Allied troops. McBride is not uniquely Irish - I am a New Zealander (with an Irish name) and when I was growing up the crazy old man who lived next door was named Terence McBride, and he was a veteran of WWI.
The composer/lyricist, Eric Bogle, says he wrote this in France, and used the name Willy McBride because it provided a suitable rhyme for "side".
Willy McBride is Everyman, the Unknown Soldier, the Eternal Warrior, the Ultimate Sacrifice. Please stop arguing over him - he's all of our sons, and brothers, and fathers, and uncles who ever joined up out of a sense of duty and a desire for adventure, and died miserably in the mud and blood of a battlefield..