De Lacy died in 1241, and through the female line the castle passed into the hands of Roger Mortimer, who landed in Ireland in 1308 and took possession.
Upon the invasion of Edward Bruce, Mortimer fled to Dublin, and Lord Walter Cusacke occupied Trim.
Orders for repairing the great hall were issued in 1326, and it was most likely at this time that the two arches which formerly crossed the northern division of the keep were erected, for when they fell in 1820 the plaster showed they had been added after the walls were built.
In 1330 Edward III. granted to Roger Mortimer, Earl of March, the moiety of Meath and the privileges exercised in Trim Castle. He was hanged as a traitor the same year, but Trim was restored to his widow.
Roger, Earl of March, was killed by the Irish in 1398, and the next year Richard II. landed at Waterford to avenge his cousin’s death, with Humphrey, son of the Duke of Gloucester and Henry (afterwards Henry V.), son of the Duke of Lancaster. The boys he brought with him as hostages, and upon returning to England he left them as prisoners in Trim Castle. A very small steel spur, inlaid with silver, was found in the castle yard in 1836, which may have belonged to one of the princes.
In 1400 Henry IV. appointed a custodian to the late Earl of March’s records at Trim and elsewhere. At this time the castle was in his hands owing to the minority of the heir.
The Privy Council in England (1403) notified to the King that the castle was in a most dilapidated condition.
From this time forward Parliaments were continually held at Trim, while in 1418 the Earl of Kildare, Sir Christopher Preston, and John Bedlow were imprisoned in the castle.
Four years later Richard Talbot, Archbishop of Dublin, resided in the fortress, and in 1425 Edmund, Earl of March, died there while Lord Deputy.
But, perhaps, the most palmy days of Trim Castle were in 1449, when Richard, Duke of York, came to Ireland as Lord Lieutenant, and held his court there.