Clackmannan, Munster, Mourne, Monmouth all appear to have the same derivation, it has been argued quite plausibly that Mann is an old Celtic word for homeland or heartland, the list of kings given in some histories would appear to be a Welsh Dynasty (Princes of Gwynedd rather than Kings) that has somehow become transposed to the Isle of Mann. When researching a project around twenty years ago I was led astray by this dynasty.
Many traps for the unwary. Merry Christmas to All.
Rick