Casement, Caseman
Casement, Caseman—This is the anglicised form of the Isle of Man name, Mac Easmuinn. This name is of Norse-Gaelic origin, its earliest form being Mac As-mundr. It was written Mac Casmonde in 1429, and Mac Casmund in 1511. The Casement family of Co. Antrim came from Ramsay, Isle of Man, about the beginning of the 18th century.
Alphabetical Index of Anglicised Surnames in Ireland
See also Woulfe’s Irish Names and Surnames
… and O'Hart's Irish Pedigrees