In case this is of any interest to anyone. I have an ancestor, Thomas Boyle, b 1821 who married a Margaret Casslin, 22 Sept 1856 at Annaduff Leitrim,.
At Mohill, Ardagh, Leitrim, on 21st July 1863: Bapt. of Thomas Boyle, Parents. Thomas Boyle and Margaret Cassills. sponsors. James Cassells and Catherine Boyle.
While trying to find her ancestors I came across so many different spellings of this name, or similar names, in the Irish Catholic Registers. but never found a likely parent for her..
The names I did find were: Cassills, Cassells Casells, Castles, Castels and Caslin.