His mother was Margaret Plunkett, daughter of Oliver, 4th Baron Louth; through his grandmother, he was a member of the powerful Bagenal family.
As a younger son he did not expect to inherit the title or estates; like many young men in his position he read law, studied at Gray's Inn, and then resolved on a military career.
In 1645 he tried to get the Confederation of Kilkenny to support King Charles I in the English Civil War on the grounds that their demands for full civic rights to be restored to Roman Catholics would be met.
During the Commonwealth he seems to have played a careful double game - his second marriage into the Holles family put him firmly in the Parliamentarian camp, and enabled him to hold on to much of his property, apart from Merrion Castle itself, but he was also suspected of working for the Restoration of Charles II.
Eventually, the Privy Council of Ireland, at the King's request, granted him a full pardon for any crimes he had committed during the Cromwellian era.