He was born in County Fermanagh, the son of Sir Bryan Maguire who was created a peer on account of the family loyalty to the English crown.
In Dublin, during the session in February 1641, he was recruited by rebel Rory O'Moore, who had conceived the plan of freeing Ireland while the English government was busy with Scotland.
In August 1641 he first heard of the plan for seizing Dublin Castle; the chief hope of the conspirators rested on Colonel Owen Roe O'Neill, who served the king of Spain in the Low Countries.
Roger More escaped, but Maguire was captured, with MacMahon and Colonel Reade (afterwards Sir John and gentleman of the bedchamber), who had served the king in Scotland.
The two latter were racked, but Maguire admitted all the material facts without torture on 26 March 1642, and made a fuller voluntary statement some six months later.