Following their defeat at the Battle of Kinsale, Cormac remained loyal to his brother when most of his other Gaelic Irish changed sides and made peace with the Crown.
Following the Burning of Dungannon, in which Tyrone destroyed his own capital, they fought a guerrilla war, and Cormac was able to ambush a force led by Henry Docwra.
When Hugh O'Neill fled Ireland in 1607, Cormac remained behind, riding to Dublin to inform the authorities of his brother's departure, and claiming he had no part in.
His claims however were contradicted by the fact he had learned of his brothers intentions at Dunalonge, being only five miles from the garrisons of Derry or Lifford, both near Lough Swilly where the earl departed.
[6] One son, Bryan Crossagh O'Neill, was granted a small proportion of land, consisting of 1,000 acres, in Dungannon barony as part of the Plantation of Ulster.