Fred there were various William McBrides who died in the war. Several of whom died in 1916. None of the gravestones match the details in the song as can be checked on the Commonwealth War Graves site. The grave you mention doesn't name his as William as it only has an initial and doesn't mention what age he is. As far as I have read Bogle wrote the song in a hotel room in Germany whilst touring with Hamish Imlach. He was probably doing it from memory but for some reason had remembered the name William McBride (perhaps simply because it rhymed with graveside) so don't you think rather than the song being about one actual idividual it is more likely to be about all the fallen. As per the unnamed warrior at the Cenotaph? The details in the song probably have more to do with what rhymed and scanned rather than hard facts!