I don't know any Welsh, Manx, Gaelic or Cornish so am out on a limb, but...

Placenames were transmitted phonetically before being written down. We know that the Romans called Anglesey "Mona", but why? If it was the local British name then it would be the Latin form of "Mon". The Isle of Man is often referred to simply as "Man". How old is that name and what is its origin? The really important thing is - how are these words pronounced because they could be identical? Forget the modern spelling. If, as someone has said, Ynis Mon is pronounced as "inis morn" then how is "Man" pronounced locally? Man and Mon could be the same.

As an aside - years ago when I worked in a library, a guy from Belfast asked me for a book on the Earl of Marne. I didn't know where Marne was and who was its Earl. I sought high and low and it was only as he grew more and more annoyed and repeated "the Earl of Marne" over and over again that I twigged that he was saying "the Isle of Man" ;o)