[1] His father was killed in 1563, but he was considered too young to inherit and the clan's leadership passed to the chief's surviving brother Eoin, who was confirmed by Dublin Castle administration with the title Lord of Beare and Bantry.

[citation needed] By 1600, the province of Munster had been devastated by battle, and Irish Catholics had lost over half a million acres (4,000 km2) of land to Protestant settlers following the defeat of the Desmond Rebellions.

According to Philip O'Sullivan Beare, Carew's men killed all 300 occupants of the stronghold, including women and children who had taken shelter there, in what became known as the Dursey Island massacre.

The march was marked by the suffering of the fleeing and starving O'Sullivans as they sought food from an already decimated Irish countryside in winter.

They faced equally desperate people in this, often resulting in hostility, such as from the Mac Egans at Redwood Castle in Tipperary and at Donohill in O'Dwyer's country, where they raided the food store of The 10th Earl of Ormond.

On their arrival at Brian Oge O'Rourke's castle in West Breifne on 14 January 1603, after a fortnight's hard marching and fighting, only 35 of the original 1,000 remained.

O'Sullivan, like other members of the Gaelic nobility of Ireland who fled, sought exile, making his escape to Spain by ship.

His nephew, Philip O'Sullivan Beare, was important in this regard and his 1618 disquisition in Latin, A Briefe Relation of Ireland and the diversity of Irish in the same, was influential.

Donal Cam O'Sullivan Beare is mentioned in the early 20th-century Aisling poetry of Seán Gaelach Ó Súilleabháin (1882–1957).