Tom,

There seems to be some form of confusion here as to if Calwell is Irish,it is not.Most Calwells who appear as you correctly say have obvious Scottish connections.However it is possible that some of the Campbell clan known in Ireland as Mc Cawell and Mc Cowell assumed the shortened version or an Irish Houriskey equally assumed it.These Campbells arrived as gallowglasses during the Elizabethan wars.But when your dealing with a basically illiterate peasant/yeoman whose spelling of his name would not be high on his survival rating,I doubt the D meant a lot to him even if he knew how to spell his name.
My research has found only "John Caldwell" of Enniskillen and Andrew Caldwell of Dublin in the early period 1614-1650 of the settlement as spelling the name in its modern form.All other spell it Calwell,Colwell and Colewell.

I checked the record for the Andrew Calwell and Alexander Colewell who in fact got the grant of a quarter (35 acres) of James cunningham 1.11.1614 of Moyegh, precinct of Portlough, not of John Cunningham of Crawfield/Cranfield in 1608 or 1610 in Ayreshire.This James is not Sir James Cunningham,but could be his uncle.However there were two James's one who got a grant of 1000 acres at Moyegh in Portlough and the other Sir James of Glangarnocke who got a grant of 2000 acres.John Caldwell did get a grant of 1000 acres in Dunboy was a son of Sir James Cunningham of Glengarnock.Even this family had a pproblem in the spelling of their name.The name was mis-spelt,Cunynham,Cunningham and Cunyngham.

It should be remembered when researching in Ireland that the plantation of the six escheated counties did not evolve until post 1610
Plunkett