Castlemartin is the name of a historic house and estate, and the townland in which they sit, on the banks of the River Liffey in Kilcullen, County Kildare, Ireland. Formerly a key estate of the Eustace family, it was for many years the home of media magnate Tony O'Reilly, and his wife, Chryss Goulandris, but was bought in 2015 by John Malone, an Irish American. The estate includes major stud farm and cattle breeding operations, a restored medieval church and an icehouse.

Kilcullen, formally Kilcullen Bridge, is a small town on the River Liffey in County Kildare, Ireland. Its population of 3,815 at the 2022 census made it the 13th largest settlement in County Kildare. From 2002 to 2011, it was one of the fastest growing towns in the county, doubling its population from 1,483 to 3,473. It is situated primarily in the Barony of Kilcullen, with a part in the Barony of Naas South, and subsidiary areas include Logstown, Harristown, Carnalway and Brannockstown, Gilltown, Nicholastown, and Castlemartin.