The king/bishop of Cashel, Cormac mac Cuilleanáin, was buried in the graveyard at St James’ Church after he was killed at the Battle of Ballaghmoon in AD908.
Up to half of the towns people were killed by the Black Death in 1348/9. In one of the later outbreaks in 1408 the Deputy Lord Lieutenant of Ireland, Stephen le Scrope, died here.
The first recorded Parliament in Ireland was held here in 1264. During this period Tristledermot (as it was known) was one of the principle towns in Leinster. The Parliament building was located on the Market Square, where the Leinster Arms pub is now.
After the Battle of the Boyne in 1690, King William of Orange stopped overnight in Castledermot on his way south. While here, he learned of the devastating defeat of his fleet in the English Channel at the Battle of Beachy Head.
George Graham (1827-1911), son of the local postmaster, went on to become a poet, a leader of the New Zealand temperance movement, and mayor of the South Island town of Waimate.
Saint Laurence O’Toole (c.1127-1180) was born near Castledermot at Mullaghreelan. He went on to become abbot of Glendalough and then the first Irish Archbishop of Anglo-Norman Dublin.
The massive north window in the Franciscan Friary was pulled down by the landowner at the end of the 18th century as he was afraid it was going to collapse on his cattle.
The famous Antarctic explorer Sir Ernest Shackleton (1874-1922) was born at Kilkea House, just outside Castledermot. As a child he was nicknamed ‘Mr Lag’ as he used to lag behind his siblings when out on walks in the locality.
The first Charter School of the Society for Promoting English Protestant Working Schools in Ireland was established here in 1734 on the site of the Castle Inn. It was suppressed in 1831 amid accusations of abuse and exploitation.
During the Bruce Invasion Castledermot was attacked twice by Edward Bruce, in 1316 and 1318.