Perhaps it ought to be recalled here that the baronies as such did not come into existence until 1570 (the time of Queen Elizabeth 1 and of the growing Reformation).
The territory indeed existed and was divided up unto little kingdoms (or tuatha), each with it’s own Irish or Anglo-Norman chief. But then in 1570 it was decided to divide up Connacht into proper counties (like the English shires) and into baronies. We can read about this division in the State Papers for that year, and happily in his book on the history of the county, Hubert Thomas Knox published the section dealing with Mayo.
Interestingly, there we find that the first recorded name of the barony of Costello was ‘Bellahaunes’ (which is near enough to the sound of Ballyhaunis). It also tells us that a MacCostello ruled the barony and that in it there were five main castles - respectively at Bellagharee, Castlemore, Turlaghane (or Tulrohaun), Annagh (presumably near Lake Mannin) and Ballyhaunis.
In any case, other early recorded names for the barony are Mac Costula, Castell-more, Clancostillo, Bellahawnesse, Mac Costillo. But the name that has stood the test of time has been ‘Costello’.
This small area of the county, long before it became a barony in 1570, long even before the Anglo-Normans came into the West around 1235, was a place of interest and activity. This was even true of the small part of the barony around Ballyhaunis (and especially as it spanned out from Mannin Lake) - here we find plenty of proofs of the existence of people from the earliest times.
From the megalithic pre-historic burial places, from the ringforts (raths, cashels and so on), from the objects found (like the Annagh sword), from the ogham stones to the remains of a patrician church at Kiltullagh (just a little outside the barony), one can trace people here from at least the Neolithic (New Stone) Age up to the coming of Christianity (and the rough beginnings of recorded history).
People, then, knew this part of Mayo before 2,000 B.C., and one can follow them through the Bronze Age (down to around 350 B.C.) and the Iron Age (up to around 500 A.D.). And so, taking in the coming of the Celts and the dramatic arrival of St. Patrick, we can make our way locally through the Golden Age and the time of the Danes, and on to the coming of the Anglo-Normans to the West in the thirteenth century. St. Mary’s Abbey was not to come until a century (or maybe two) later, but since then it has marked the passing of time, and today it is often called 'an archaeologist’s dream'. But if it is, then surely one must also hold that the Abbey itself is set in a rich archeological eden.
Extract from 'St Mary's Abbey'