Cormac was born in 1552,[1] the eldest son of Dermot MacCarthy and Ellen FitzGerald.

[8] Cormac had a brother Teige, who was ancestor of the MacCarthys of Insirahell near Crookstown, County Cork, and two sisters Julia and Grainé.

[9] Cormac MacDermot MacCarthy, conformed to the established religion by adhering to the Church of Ireland.

Dermot and Mary had three sons: —and one daughter: His father died in 1570 when Cormac MacDermot was about 18 years old.

[30] According to English Common Law he would have immediately succeeded as 14th Lord of Muskerry, but as a minor his estate would have been sequestered by the crown and he would have become a ward.

However, Brehon law was applied[31] and his uncle Sir Cormac MacTeige MacCarthy succeeded in his stead, according to tanistry.

On 21 October 1601 he attacked the Spanish positions with his Irish forces[37] fighting under George Carew, Lord President of Munster.