Courcy's fall from power occurred in a series of conflicts with the English Crown between 1201 and 1204. By 1205 he was forced from Ireland altogether, and his lands were awarded to Hugh de Lacy. During Courcy's subsequent revolt within the year, he received military support from Rǫgnvaldr, his brother-in-law. The "Chronicle of Mann" specifies that Courcy's massive force was reinforced by Rǫgnvaldr with one hundred ships, and states that they laid siege to a certain castle of """", before being beaten back with the arrival of Walter de Lacy. The expedition is also recorded by the sixteenth-century "Annals of Loch Cé", which states that Courcy brought a fleet from the Isles to battle the Lacys. Although the expedition ultimately proved a failure, the source indicates that the surrounding countryside was plundered and destroyed by the invaders. The identity of the castle named by the chronicle is almost certainly Dundrum Castle, which was possibly constructed by Courcy before 1203. The defeat of 1205 marks the downfall of Courcy, who never regained his Irish lands.