Entering Trim Castle. Trim Castle (Irish: Caisleán Bhaile Átha Troim) is a castle on the south bank of the River Boyne in Trim, County Meath, Ireland, with an area of 30,000 m².]Over a period of 30 years, it was built by Hugh de Lacy and his son Walter as the caput of the Lordship of Meath. The Irish Government currently own and are in charge of the care of the castle, through the state agency The Office of Public Works (OPW).