Not as uncommon as you might think, Mick. This site says:

Gaelic is nothing new to Irish Presbyterians who trace their roots in Ireland back to 1642 when the Scottish Regiments stationed at Carrickfergus brought their Presbyterianism to these shores. They spoke both Gaelic and Lowland Scots which later became Ulster Scots. Until the early 19th century there would have been Presbyterian communities who spoke and worshipped in Gaelic in places including Bushmills, the Glens of Antrim, Ballybay and Dundalk. Until around 1850 it was a condition that students for the Presbyterian ministry undertook classes in Irish.