According to traditional genealogy, the O’Sullivans were descended from the ancient Eóganacht Chaisil sept of Cenél Fíngin, the founder of the clan who was placed in the 9th century, eight generations removed from Fíngen mac Áedo Duib, king of Cashel or Munster from 601 to 618.

Later, they became the chief princes underneath their close kinsmen, the MacCarthy dynasty, in the small but powerful Kingdom of Desmond, successor of Cashel/Munster.

According to the genealogy recorded in the 17th-century Leabhar na nGenealach, the O'Sullivan clan claimed descent from the Eóganachta dynasty of the Kings of Munster (and via them, ultimately, from Milesius, Fénius Farsaid and Adam).

The Irish laws of Tanistry required that the title of chieftain be passed on to the most capable of the dead chief's family.

The O'Sullivans and other clans provided shelter to 12-year-old Gerald FitzGerald when troops sought to capture him as the last heir to the Earlship of Desmond.

In the late 1590s, it was the Sullivan Mor clan and their close allies the McSweenys who bore the brunt of the fighting with the English forces.

As retribution for their support of the Desmond rebellion, the Munster clans lost over 500,000 acres (2,000 km2) of their land to English settlers.

King Philip III of Spain agreed to send help to his co-religionists in Ireland under the command of Don Juan D'Aquila.

The war weary and decimated Munster clans had difficulty mustering an army to join the Ulster and Spanish forces.

The Spanish were given the responsibility of forming the garrisons for the castles of the O'Driscolls and the O'Sullivans so as to free the Irish troops for the battles to come.

Donal O'Sullivan Beare was given command of the Munster forces, which consisted mainly of soldiers of his clan and those of the O'Driscolls, McSweeneys, and O'Connor Kerry.

This was due in large part to the reluctance of the Spanish troops to leave the protection of the walled city of Kinsale and join the battle until it was over.

General Aquila sued for peace and Lord Mountjoy, commander of the English, was only too happy to accept his request.

Dónal O'Sullivan and approximately one thousand followers consisting of four hundred soldiers and the rest civilians began a journey to Leitrim to the castle of his friend Ó Ruairc (O'Rourke).