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 castle is partly covered with ivy.
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.
It was tenanted by a Mr. Isaac Dobson during the reign of Charles II. He was a Nonconformist, and probably a trader in Dublin. When James II. came to the throne he left the country, and was attainted by Parliament in 1689.
His son, who was a bookseller, succeeded him at the castle. He greatly improved the grounds, and when he died in 1720 he left the use of the castle to his wife for her life, after which it went to his sons.