This famous and epic march undertaken by O' Sullivan Bere to Leitrim through the Counties of Cork, Limerick, Tipperary, Offaly, Galway, Roscommon, Sligo and eventually Leitrim arose as a result of the Irish and the Spanish defeat in the Battle of Kinsale at the hands of the Crown forces. The Battle of Kinsale began on the 17th of October 1601 with the 3400 Spanish soldiers under the command of Aquilla supported by O'Sullivan Bere, O'Driscoll and the O'Connors of Kerry. Aquilla surrended on the 12th January 1602 and handed over the four Spanish defended castles along the South West Coast. Donal Cam, chieftain of the O'Sullivan Bere Clan rushed back to Dunboy and began to fortify the castle against an attack that started on June 6th and lasted 11 days with the Crown forces storming the castle and bombarding it with cannon-fire.

Harassed by the Crown forces and having lost his lands and his herds of cattle and sheep, he left the Beara peninsula and the Bay of Bantry where the French invasion took place in 1796, to begin the long march to Leitrim to meet the O'Rourkes. Accompanying him were 1,000 men, women and children representing the first large-scale exodus of people from Beara peninsula region. When the Beara men travelled from West Cork as well as his followers were members from other clans O'Collins, McAuliffe, O'Rourke, Fitspatrick, McGuire, O'Keeffe, O'Donoghues,O'Driscoll, McCarthy, McSweeney, Carrs, Quigleys and Naughtons. In the middle of January 1603 they finally reached their destination with only 35 people remaining, many settling along the route and been known since then in these localities as the Bearas.

After Dunboy (the home of the O'Sullivan Bere Clan) and Dursey Island castles and their defenders were wiped out in June 1602, O'Sullivan Bere went on a campaign of guerrilla warfare around West Cork, where he took at least six castles. By December 1602 Donal Cam and his followers were camped for the winter in Glengarriff. Crown forces, under Wilmot, attacked and took his creacht, 4000 cattle and 2000 sheep. Faced with overwhelming odds and starvation and the O'Sullivans left Beara on their epic march to Leitrim.