There have been several periods since the early days of the Gaelic League, when sets of Irish words have been consciously produced to popular tunes from other cultures/countries - including Nazi Germany, but that's another story! I've never heard a set to "Johnny...".

Regards

P.s. At a concert in a yacht club near Galway a few years ago, I heard a shanty called "Rolling homeward to Oranmore" . Next day, as I queued at a supermarket checkout, I was subconsciously humming the tune as I tried to remember the words... "My uncle wrote that!" Said the checkout girl! He wrote "an Cailín Álainn"!