Castletown Geoghegan (Irish: Baile Chaisleáin Mag Eochagán) in County Westmeath, Ireland, lies south west of Lough Ennell near the county town of Mullingar. Castletown was the seat of the Geoghegan family of the medieval Barony of Moycashel, County Westmeath. The family were descendants of the Southern Ui Neill. They were major landholders in south Westmeath prior to the Cromwellian invasion, the Down Survey and ensuing Plantations that followed that and the later War of the Three Kingdoms. They lost considerable portion their estates to Gustavus Lambart (later created Earl of Cavan) through confiscation even prior to the publication of the Down Survey. The abundant productivity of the rich grazing being so self-evident he contrived to procure it early in the piece.