Two large windows on the ground floor also point to later alterations, especially as they occur simultaneously with the remains of earlier openings.
The stairs are likely to have been situated in the south-west side. There are numerous small chambers and passages in the thickness of the walls.
Of the three fireplaces in the north-west wall that on the ground floor is the largest, measuring 9 feet long by 5 feet high, and as the flagstones of the hearth are covered by some inches of gravel its height was once greater. At the back of the fireplace and slightly to one side is an aperture about two feet square, which is framed in cut stone, and was probably used as an oven.
On the south-west end of the keep are the ruins of a smaller building several storeys high, which is connected with the main building by a square topped doorway. The dividing wall is nearly 6 feet in thickness.
Both buildings are largely covered with plaster, and on the south-east the keep has been partly rough cast.
The situation of a third building can be seen adjoining the tower on the north-east side, where the pitch of its roof may be traced about three-fourths of the way up, but very little of the walls remain.
The fortress seems to have been built soon after the Norman invasion, and as the lands of Dundrum were held by Hugh de Clahull, it was probably erected for their defence. It subsequently passed to the Fitzwilliams of Merrion, from whom it descended to the Earls of Pembroke, and it forms at present part of the Pembroke estate.
Robert le Bagod, ancestor of the Fitzwilliams, had license to convey the manor of Dundrum to his son William, and in 1332 Thomas Fitzwilliam was found seized of the lands round.
From this time on it is likely the fortress was occupied by cadets of the Fitzwilliam family.
In 1542 Sir Thomas Fitzwilliam was in possession of the manor, and in 1616 his grandson Thomas, afterwards first Viscount Fitzwilliam, recovered the lands of Dundrum and Ballinteer, with the castle and water mill.
His brother William, who had married Archbishop Ussher’s widow, lived at Dundrum Castle about this time.
The building was slated and in good repair during the Commonwealth. It is stated to have had three hearths and a barn, with a garden.