John Mor MacDonald married Margery Byset, daughter of the Mac Eoin Bissett, Lord of the Glens of Antrim. Although the MacDonnells would a century and a half later claim that her dowry included the lordship itself, this is false because the Mac Eoin Bissetts remained in possession of it into the early or mid 16th century. However following the death in battle, in support of his friends and kinsmen the O'Neills against the O'Donnells, of the last known Mac Eoin Bissett in 1522, the recently arrived MacDonnells appear to have started making their own gains in the region and eventually used the earlier marriage to claim they had rights to the Bissett lordship. The MacDonnells already possessed Dunnyvaig Castle on the Isle of Islay and lands in Kintyre, Scotland.