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,[2] and his wife, Chryss Goulandris, but was bought in 2015 by John Malone, an Irish American.

Accesses to the main body of the estate, secured with gates and cameras, are from Newbridge Road, on the approach to Kilcullen from the Curragh.

The house is located close to the Liffey and is reached from the main gates by way of a lime avenue, with two trees joining to form an arch at one point.

Having fallen into ruin, St. Mary's was restored by Tony O'Reilly between 1979 and 1980, under the supervision of former Inspector of National Monuments Percy Le Clerc.

[2] This, the first restoration of a medieval church in Ireland, was authorised on the basis of close adherence to original forms and ongoing reasonable public access.

In the National Inventory of Architectural Heritage it is described as follows:[4] This gateway, possibly originally fashioned for installment at alternative location, is an attractive, highly ornate composition that provides a fitting entrance to the Castlemartin (House) estate.

The piers, gates and railings are fine examples of early surviving wrought iron work and include intricate detailing, which is of artistic interest.

The gateway is a prominent and attractive feature on the side of the road leading out of Kilcullen to the west.The estate lands of around 750 acres (3.0 km2)[3][2] are a mix of parkland and working areas, crossed by avenues and watercourses.

Entries in the National Inventory of Architectural Heritage for Castlemartin note the survival of sections of wrought iron railings from around 1830, on rubble stone boundary walls.

[3] Under O'Reilly ownership, the stud farm was part of a structure which held animals locally as well as in France, England, Australia, South Africa and the US, had 155 broodmares (at three locations) and 105 horses in training at the beginning of 2008.

O'Reilly, however, spent millions of pounds on repairs and improvements to the Georgian house and its outbuildings, and on the restoration of the estate church, icehouse and other features.

[3] It was sold to John C. Malone, billionaire owner of Liberty Media, with the agreement made in December 2014, and closure in January 2015 for around 28 million euro.