It is generally supposed that Dundrum Castle was built by John de Courcy at the end of the twelfth century for the Knights Templars, after his daring conquest of Ulster in 1177 with only a force of about a thousand men. The stronghold remained in the possession of the order (which was bound by vows of chastity, poverty, and obedience) until the suppression of the Knights Templars in 1313. It then passed into the hands of the Prior of Down, and is mentioned by Archdall in his “Monasticon Hibernicum” as a religious house. Upon the abolition of the monasteries the reversion of the castle and manor, with a yearly rent of £6 13s. 4d. reserved out of it, was granted to Gerald, Earl of Kildare. In 1516, however, it appears to have been in the possession of O’Neill, who fortified it, with a boast he would hold it against the Earl of Kildare, at the same time sending to the King of France to come and help him to drive the English out. The following year Gerald, 9th Earl of Kildare, and Lord Deputy, marched into Lecale and took Dundrum by storm, but it seems almost immediately to have reverted to the Magennises, who repaired it. In 1538 it was retaken along with seven other castles by the English, commanded by Lord Deputy Grey, who says: “I took another castell, being in M’Geeon’s countrie called Dundrome, which, I assure your lordship, as it standeth is one of the strongest holds that ever I saw in Ireland, and most commodious for defence of the whole countrey of Lecayll, both by sea and land, for the said Lecayll is invironed round about with sea, and no way to go by land into the said countrey but only bye the said Castle of Dundrome.” After this the castle appears to have remained in the hands of the Crown for a few years. In 1551, we learn from the records of the Privy Council that Prior Magennis was seized and imprisoned in Dundrum Castle by Roger Broke without order of law. Six years later Lord Deputy Sussex asked that Lecale with the Castle of Dundrum might be granted to him in fee-farm for ever. But again in 1565 it was occupied by the great Shane O’Neill, who placed his own ward in it for defence, and the Magennises (with whom O’Neill was intimately connected) were in possession of the stronghold in 1601, when Phelim Magennis surrendered it to Lord Mountjoy. O’Neill is said to have been a constant visitor at the castle while it was possessed by the Magennises, Lords of Iveagh, and after a night of revelry would indulge in a strange kind of bath, by being buried to his neck in the sands on the shore of the bay. Four years subsequently to the stronghold passing into the hands of the Crown, Lord Cromwell was commissioned to be governor and commander of Lecale and the tower and castle of Dundrum. In 1636, Lord Cromwell’s grandson, Thomas, Lord Lecale and 1st Earl of Ardglass, sold it to Sir Francis Blundell, from whom it descended by marriage to its present owner, the Marquis of Downshire.