Thanks for the corrections. That is why the Google can't be trusted. I should know better.
Several websites mention Kavanagh.
I just checked folktrax and found the O Lochlainn 1939 reference (Roud 2999), cited by Big Tim. It has the line "Johnny went a-thrashing the dirty King of Spain," so seems to be a little different from those in the DT.

What were the words in O Lochlainn's book of 1939? (Irish Street Ballads?)

Any prior records? Not in the Traditional Ballads Index. Not in Sam Henry.