The youngest brother of Red Hugh O'Donnell, Cathbarr fought in the Nine Years' War.

Born c. 1583,[2] Cathbarr was the fourth and youngest son[3][4] of Sir Hugh McManus O'Donnell, an Irish Gaelic lord who ruled Tyrconnell from 1566 to 1592.

[4] His elder brother Hugh Roe was a key confederate leader during the Nine Years' War (1593–1603).

In 1599 he fought alongside his brothers at the Battle of Curlew Pass, which resulted in a crucial victory for the Irish confederacy.

[12] In early July 1608, Cathbarr travelled to Ostia, a coastal town fifteen miles west of Rome, in order to "make holiday and take a change of air".

He was accompanied by his brother Rory, Hugh O'Neill, 4th Baron Dungannon, and Donal O’Carroll, Vicar General of Killaloe.

[12] After Cathbarr's death, Rosa remarried to the Irish soldier Owen Roe O'Neill.

[15] Cathbarr had a son with Rosa named Hugh O'Donnell, who was aged two years and three months at the time of the Flight of the Earls in September 1607.

[19] He was later imprisoned in a London prison, and then in 1629 escaped to Flanders alongside his cousins Mary Stuart O'Donnell and Hugh O'Rourke.