We cannot understand those times by simplistic phrases like "descendants of the Gael" and "supplanters". Some of the most ferociously Protestant (in Ireland and Scotland) were as gaelic speaking and Gaelic descended as you like. And poltical alliances did not necessarily follow religious or ethnic lines either. The first earl of Inchiquin undoubtedly sided with Cromwell and killed a load of Catholics, but he killed plenty of Protestants too. And he was a perfectly Gaelic O'Brien. The religious ferment of the 1600's was not something Ireland was immune to: it didn't import all its Protestants, there were plenty of homegrown ones. The intimate linkage of Roman Catholicism and Irish nationalism was a much later historical phenomenon.
    Carolan was a harpist, towards the end of a long tradition, who lived on the patronage of the nobility of the time. The fact that he was personally a Catholic, and some of his patrons were Protestants, is hardly surprsing.