By the 1630s he had established a thriving legal practice: the attempts by Thomas Wentworth, the Lord Lieutenant of Ireland, to confiscate as much Irish land as possible to the Crown, ensured that his services were in high demand.
However, in mid-1642 government troops looted and torched his home in Balrath, County Meath: Plunkett unsurprisingly thereafter gave support to the leaders of the Irish Insurgents.
Only six months later the New Model Army launched a full-scale invasion of Ireland, and within a year had conquered large areas of the island.
Ulick Burke, 1st Marquess of Clanricarde was appointed leader of the Irish forces in late 1650, replacing the discredited Earl of Ormond.
Nicholas Plunkett's brother, Patrick, was also prominent in Confederate politics, becoming Bishop of Ardagh in 1647 largely thanks to the backing of Rinuccini.