Standing three storeys high beside the Phoenix Park Visitor Centre, this fortified structure dates from around 1605 and represents a fascinating slice of Dublin's turbulent past. The castle's lands originally belonged to the Hospital of St John Without, Newgate, following a 13th-century grant by Hugh Tyrrell, First Baron of Castleknock, before eventually passing through various hands during Ireland's religious and political upheavals.

The castle's most colourful owner was undoubtedly John Connell, a 17th-century seneschal to the Duke of Ormond who earned himself quite the reputation. Lord Orrery branded him a 'notorious rogue and tory', and Connell eventually forfeited his extensive lands in Kerry and Dublin, including portions of what would become Phoenix Park. The 1654-56 Civil Survey recorded that this Protestant landowner possessed 'one Castle with two Thatcht houses' valued at eight pounds, along with a small orchard. After Connell's forfeiture in 1664, Sir William Flower purchased the estate and became the park's second keeper, beginning a new chapter in the property's history.

Today, visitors can explore this rectangular limestone tower house with its projecting stair turret in the southeast corner. The structure retains many original features including musket loops for defence, blocked doorways that hint at altered access routes, and the remnants of lintelled fireplaces on multiple floors. An early 17th-century roof truss in the east gable helps date the building's construction, whilst Georgian windows inserted into the ground floor serve as reminders of its later domestic modifications. The surrounding landscape, laid out by Ninian Niven around 1840, includes mature gardens and low box hedging that marks the ground plan of the demolished Ashtown Lodge, creating a palimpsest of different historical periods in this corner of Phoenix Park.