The buildings housing the school were erected in 1771 in the Phoenix Park, overlooking the village of Chapelizod in the Liffey valley (in full view of the Wicklow mountains). The chapel was designed by Thomas Cooley, while thirty years later Francis Johnston designed the extensions to the buildings. It first took in 90 boys and 50 girls as pupils (in the charge of an Inspector and Inspectress, assisted by the Chaplain and an assistant mistress) in March 1770. The site originally occupied but by 1922 its boundary walls enclosed thirty three acres.