Now despite references in documents and so on, it is difficult to pin-point when the town of Ballyhaunis was founded.
One usually reads that it grew up around the Augustinian Abbey (when it came into being either in 1348 or around 1430). That would seem to be the accepted view, and without a doubt the existence of an Abbey would inevitably have drawn houses around it, and so could have the kernel of a town or village.
However, perhaps a case could be made that some such town already existed and that Ballyhaunis has roots that go back beyond the 15th or the 14th century, back beyond the founding of the Abbey. But to prove that, of course, is another matter. One fruitful avenue might be to investigate more thoroughly the take-over from the Irish by the Normans in this area around Ballyhaunis. A paper-cutting mentioned a Fitzgerald fortress and manor on the Abbey Hill – that would seem to have been a tradition, and I understand that it was expressed in writing in the 17th century (in a preparation for a map that was published in 1685). Well, if that manor existed in fact, there would almost certainly have been some kind of town, as the barons encouraged trade around their places. And where there is trade, there is a town.
Another line of argument for a pre-existing town (prior to the building of the Abbey) could be based on the custom of the friars themselves. Unlike the monks, who built in remote places far from people, the friars usually settled where people already existed in communities (or where people passed by in goodly numbers, as for instance, on pilgrimages). So one could lean towards the view that a town did indeed exist in Ballyhaunis before the Abbey was built. However, even if it did, it would surely have become much larger and more significant because of the presence of the friars - and what may have been a mere hamlet around a castle could then well have become an important town.
All this, of course, is a subject for study. One thing we can be fairly sure of, I think, is that the town did not exist in the time of St. Patrick. Of course, St. Patrick and his contact with Connacht are matters for debate. But if one follows Monsignor Dalton’s history of Tuam, one will accept that about 440 the saint established a church on the shores of Lake Mannin and that he then went "due south, passing through the site of the modern town until he reached Holywell." The town in question is Ballyhaunis, which came into being either with the foundation of the Abbey, or sometime previously at some unknown date.
Ballyhaunis is not shown in either the earliest map of Mayo (published in 1585), or in the 1685 Sir William Petty map of the county. But this is not surprising, as in neither is much attention given to the whole barony of Costello. However, we do know from other sources that even at the time of the first map, both a castle and an Abbey did exist at Ballyhaunis, and they certainly speak for a significant identity.
Extract from 'St Mary's Abbey'