Hello Sue,
I don't think I will be able to throw much light on this, maybe after I have studied it some more.
CR on her Callow trees does not have an Isabel married to a William Kerruish but on the Ballaglass tree there is a possible candidate.
Isabel 1st,d. and 4th child of Christopher c1660-1717 m. Margery Christian d.1734 Lewaigue. Christopher was the son of William Callow Ballagilley d.1676 and Margery Kerruish d.1687 William brought Ballaglass from his niece another Isabel d. of Joney Callow and Patt Corkhill-Ballaglass. This Isabels brother William who married 1661 Joney Kerruish, died before her in 1669.
I hope this may be useful to you and thank you for the other information on Kerruishes it may change my perception of my line.
Rick