The rule of Leopoldo O'Donnell was marked in 1844 by a cruel and bloody persecution of negroes for a supposed plot of servile war; O'Donnell's actions being partly due to the inquietude that had prevailed for some years over the supposed machinations of English abolitionists and even of English official residents in the island, and also over the mutual jealousies and supposed annexation ambitions of Great Britain and the United States.
O'Donnell's pronunciamiento in 1856 put an end to the Cortes, and the militia was disarmed, after a sharp struggle in the streets of the capital.
In 1542 he went to England and presented himself, together with Conn O'Neill and other Irish chiefs, before Henry VIII., who promised to make him earl of Tyrconnel, though he refused O'Donnell's request to be made earl of Sligo.
In 1592 Hugh abdicated in favour of his son Hugh Roe O'Donnell (see below); but there was a member of the elder branch of the family who resented the passing of the chieftainship to the descendants of Manus O'Donnell's second marriage.
Niall Garve, however, went over to the English, making himself master of O'Donnell's fortresses of Lifford and Donegal.