With the coming of the Anglo-Normans to the west in the 13th century, the powerful Nangle (or deAngulo) family took over the north-western part of the future barony of Costello. A branch of the Fitzgeralds is associated with the district around Lake Mannin.
The date 1249 is mentioned for the Fitzgerald takeover in a newspaper cutting (of 1932) and it indicates that they had a fortress and manor house on the Friary Hill. This territory around Ballyhaunis came into the hands of the above mentioned Nangle family, which in it’s own time changed it’s name to MacCostello. The branch of the MacCostello family around Lake Mannin and Ballyhaunis was to be known as the MacJordan Duff (or Dubh), and its principal stronghold was to be at Rathnaguppaun. Around 1282, the MacJordan Dubh branch of the MacCostello family took over around Ballyhaunis. This family went on to rule in the area until 1586 and it was this line of the Costellos that built the Augustinian Abbey.
The date that appears at the entrance to the Abbey is 1430 and this would be in keeping with Augustinian records in Rome. For the year 1432, these records mention an Irish Augustinian house that was ‘recently founded’. To strengthen the argument, there is a Friar Philip Nangle associated with the foundation. The Ballyhaunis house is the only Irish Augustinian foundation associated with this family.
Extract from 'St Mary's Abbey'