Olave, son of Godred Crovan, began his reign over Mann and the Isles in 1114 CE. He married Affrica, the daughter of Fergus, a chieftan or prince of Galloway. He had a son by Affrica named Godred who came to the throne as Godred III in 1154 CE. Some of Fergus’ kinsfolk settled on the Isle of Man, mostly in Kirk Maughold, and we find a descendant of one of these, John MacFergus in the Keys in 1417, almost 600 years ago. The family did not supply another member for nearly five centuries, when another John – John Robert Kerruish – became a member in 1879.
Let us trace the name from its beginning. Its primitive Celtic form was Vergustus, meaning ‘super-choice’ or ‘super-selection.’ This became in Old Irish ‘Fergus’ with a genitive ‘Fergosso.’ By aspiration or softening of the medial ‘g’ the name became ‘Fearghus’ (gh is silent) in Middle Irish. It belonged to that class of nouns called heteroclite, i.e. belonging to more than one declension. In the first declension its genitive was ‘Fearghuis’ (pron. Ferroosh), and this is the form we find our Manx name. When a name was prefixed by ‘mac’ (son), not only was the following personal name put in the genitive case but the initial consonant was aspirated, and when ‘f’ is aspirated it is not sounded at all, thus, instead of a form Mac-Ferruish we get MacKerruish, and later Kerruish.
If a personal name happened to resemble in sound a word or more of the spoken language, then popular etymology usually had a hand in the game, causing further departure from the original form. Thus the name we are dealing with resembled two Manx words: ‘kiare rooisht’ (four naked). A legend arose that there occurred a shipwreck on the coast of Maughold, four men escaped to the shore, with not even the proverbial barrel to cover their nudity, and these four men were the founders of the Kerruishes of Maughold.
(source: IOM Natural History and Antiquarian Society Proceedings Vol. IV No.1 (1932-1935))