As Martin Ryan has pointed out, a number of songs are interpreted by some Irish groups as having an element of Irish nationalism or anti-English feeling even when there's nothing much in the words, or indeed the intentions of the writer, to justify that.
The song, as you'll see elsewhere, is not called Green Fields of France, except by lazy singers who can't be bothered to get their attributions right. Eric Bogle who wrote it called it No Man's Land. The folk process does alter titles and lyrics, but when the author of a song is still alive, most people regard it as courteous to give him credit and try to get things more or less right. He's a Scotsman who moved to Australia, so he probably used the name "Willy McBride" to refer to a Scots soldier; this makes it rather an inappropriate target for anti-English sentiments, really.
Although done to death in some circles, it's still a good anti-war protest song.
Marje