Within the grounds of Howth Castle are the ruins of St Mary’s Abbey, originally founded in 1042 by the Viking King Sitric, who also founded the original church on the site of Christ Church Cathedral. See the caretaker or read the instructions on the gate for opening times.
The abbey was amalgamated with the monastery on Ireland’s Eye in 1235. Some parts of the ruins date from that time, but most are from the 15th and 16th centuries. The tomb of Christopher St Lawrence (Lord Howth), in the southeastern corner, dates from around 1470.