O'Donovan, and a number of his kinsmen, joined the so-called Irish Rebellion of 1641 under Donagh MacCarthy, along with the MacCarthy Reagh (Cormac) and the O'Sullivan Beare, and together they besieged Cork city for three weeks in 1642 with over four thousand men. But their force was opposed and defeated by the government supported Murrough O'Brien. Complaining of O'Donovan's other activities in 1642, the Reverend Urban Vigors writes: