Plunkett
I seem to recollect years ago I read a book on Irish names by Wolfe (I think). He seemed to think that there were a good number of native Irish who had their surnames anglicised when they were recorded by the English. If I remember correctly he said that the Irish surnames of Horisky and McAughwell were changed to Caldwell. If this has any meat in it there should be three different streams of Caldwells in Ireland - those originally from Scotland; those originally from England; and native Irish families with a surname change.
I have also wondered about the Manx habit of dropping the "M" in "Mc" eg: Cowan=McOwen. Any chance of McAldwell? It is easy to see McAughwell=Caughwell=Caldwell - especially when it might be pronounced Kuh-well as has been posted on this site by a US contributor.
May also help to explain the seemingly vast numbers of Northern Irish Caldwells compared with Scots Caldwells.
Only a thought - I welcome your response from the old country.
Tom