[2][3] O'Moore's uncle Rory Oge O'More, Lord of Laois, had fought against the English during the Tudor conquest of Ireland.
[4] Given the causes of the rebellion and the Crown's weakness during the Bishops' Wars into 1641, O'More planned a bloodless coup to overthrow the English government in Ireland.
With Connor Maguire, 2nd Baron of Enniskillen he planned to seize Dublin Castle, which was held by a small garrison, on 23 October 1641.
[5] O'Neill had some success, and O'More quickly succeeded in creating an alliance between the Ulster Gaelic clans and the Old English gentry in Leinster.
[6] In the ensuing Irish Confederate Wars, a major achievement by O’Moore was to recruit Owen Roe O'Neill from the Spanish service in 1642.
The Irish historian Charles Gavan Duffy wrote: Then a private gentleman, with no resources beyond his intellect and his courage, this Rory, when Ireland was weakened by defeat and confiscation, and guarded with a jealous care constantly increasing in strictness and severity, conceived the vast design of rescuing the country from England, and even accomplished it; for, in three years, England did not retain a city in Ireland but Dublin and Drogheda, and for eight years the land was possessed and the supreme authority exercised by the Confederation created by O'Moore.
[7] Bishop Michael Comerford wrote that after O'More's defeat at the Battle of Kilrush in April 1642 he retired and died in Kilkenny city in the winter of 1642–43, having co-founded the Irish Catholic Confederation there a few months earlier.
He was esteemed and loved by his countrymen, who celebrated his many deeds of valour and kindness in their songs and reverenced his memory, so that it was a common expression among them; "God and Our Lady be our help and Rory O'More".Comerford quoted earlier historians who wrote that a similar watchword in Kildare was: "Our trust is in God and Our Lady, and Rory O'More".