The son of clan chief Rory O'More, he was brought up in County Wicklow by his maternal uncle Fiach O'Byrne.

He had six full-siblings, which include Fiach, Remainn (fellow rebels) and Doryne,[1] who later married Captain Richard Tyrrell.

[1] After Rory's death, his friend John Burke, son of the Earl of Clanricarde, took charge of Owny O'More.

In response, O'Byrne dispatched his teenaged nephew, supervised by his ally Piers Grace, to terrorise the Irish midlands.

He slew a gentleman of the English, who was seated at Stradbally-Leix, who possessed a large portion of the territory by authority of the Sovereign, namely, Alexander Cosby, the son of Master Frauus.

[7]On 7 December 1597, O'More and his allies Tyrrell and Nugent killed Walter Hovenden in battle, defeating his troops.

[9] One of O'More's most significant battles, the Pass of the Plumes, took place on 17 May 1599 against Lord Deputy Robert Devereux, 2nd Earl of Essex.

[12][13] According to Geoffrey Keating's History of Ireland, "in the year 1599, Owny MacRory O'More cut off a great number of the troops of the Earl of Essex, in a defile in their progress through Leinster, at a place called from that circumstance Bearna-Cleitigh, signifying the Pass of Plumes, from the great quantity of plumes left there, which were worn in the helmets of the English knights who were slain.

[1] According to writer Philip O'Sullivan Beare, O'More "incautiously advanced with one comrade beyond his own troops [and] was struck by a leaden bullet.

[1] His first cousin, born around the time of his death, was Rory O'Moore, a principal organiser of the Irish Rebellion of 1641.