Today, the name of Ballyhaunis sounds sweet on the tongue, but down the centuries, it must be admitted, the word has had a rough passage enough. For instance, in what historians call the 'Fiants of Elizabeth' (no.3368) the name given is 'Bellahanasse,' which is extremely like a Latin name (i.e. 'Hanahannassa') that is found in Augustinian sources in Rome for the year 1432, and that is certainly an Irish place-name and is thought most likely to refer to Ballyhaunis.
The historian Knox, in quoting from a document of 1570, gives the name of the town, but almost certainly he then modernised the spelling, as we said. However, he does give the name of the barony as 'Bellahaunes' (which one may suppose was also the spelling at the time of the name of the town). But whatever about 1570, a little later (in 1586) another document gives the name as 'Bealahawnish', which I think sounds remarkably near to the name in Irish.
In any case, some forty years later (in 1635), there seemed to have been a little more difficulty with the name, for we get the version 'Ballahawnis.' By 1791 (as we know from a man named Grose), the name was being written as 'Ballyhaunes' or 'Bealahaunes.' However, certainly by 1833 (as we know from house-books preserved at the Abbey), the name was Ballyhaunis. And so in English it has remained.
The name in Irish is preserved as 'Beul Áth hAmhnuis' in the 18th century song (of that name) by the vagabond poet and Augustinian friar, Tomás Ó Caiside (An Caisideach Bán). We also find the name (with a slightly different spelling) in the popular song 'Máire Bheal Átha hAmhnais' (of which there are different versions, but the original coming, it is thought, from the same friar).
Extract from 'St Mary's Abbey'