The castle of Ballymore was in the 13th century part of the Manor of the archbishops of Dublin and in 1244 Henry III confirmed the archbishop’s right to hold a fair there. This lasted for eight days commencing on the Eve of the Assumption of the Virgin, to whom the local church was dedicated. Being on the border of the Pale, Ballymore was constantly raided by the O’Tooles and the O’Byrnes.
It was in 1373 that the name Eustace first became associated with the place when Thomas Fitzoliver FitzEustace was appointed constable of the castle with a salary of £10. The conditions were that he should reside there and guard and keep the castle. There is now no trace of the castle though the site is believed to have been on Garrison Hill, close to the ruined mill on the Liffey.