Thanks for the link, Jack. I played some versions of 'Boyne Water' as seen on abcnotation.com, and I agree that 'Boyne Water' and 'King William's March' are similar.

In fact, it's so similar that I think it's inaccurate to call it Jimmy Garson's March.

I suspect that it was given the name 'King William's March' because most people knew the tune was the same but different, and they wanted a title that also was the same but different. (The OP makes it clear that Peter Kennedy actually gave it the name.)

It's a good tune, and I might have added it to my collection, but I'm not interested in using music to promote ethnic division.