Lullymore is most famous for its monastery which historical scholars believe was the largest in Ireland in the mid 5th Century. This religious community dates back to St. Patrick who ordered Bishop Erc to found a monastery and school in Lullymore. Saint Erc was a Druid to the Ard Ri (High King) Laoighaire but was converted to Christianity by Saint Patrick. There is a large boulder on the present monastic site with a footprint distinctly impressed on it which is said to be that of Saint Patrick.
Recent archaeological surveys on the ditches indicate that the Monastic site was built on prehistoric ritual or Henge field site which could date back to stone age times. Erc was a pagan druid who was converted to Christianity by Saint Patrick and became his first bishop. Lullymore was chosen as the site of the new monastery because it was an island entirely surrounded by a sea of bog. This insulated the monks from the distractions of the world and protected them from attack. Early monasteries were often raided by pagan tribes and, later on, by bands of Vikings.
According to local tradition, the monastery was attacked in the early 18th Century and all were massacred except for one monk named Thomas Foran who is said to have escaped with important records.