As part of the Plantation of Ulster, in 1608 Lennox was granted lands at Portlough in the Barony of Raphoe in County Donegal. The Pynnar Survey of 1618 records Lennox as the chief undertaker for 2,000 acres in the Portlough area and as represented locally by his agent Sir Aulant Aula. Newtownstewart in County Tyrone, now in Northern Ireland, may have been named after him. In the Muster Rolls of 1631 his nephew and eventual heir James Stewart, 1st Duke of Richmond, 4th Duke of Lennox is described as being an undertaker of 4,000 acres. Mongavlin Castle was built by his son Sir John Stewart, who was also Governor of Dumbarton Castle.