Altogether, seventy-two civil parishes have developed in Mayo since 1570, and down the years until 1898 the areas they covered remained unchanged. However, in 1898 the county councils came into being and here and there the boundaries of civil parishes were altered. So for instance, prior to that year, there were five complete civil parishes in the barony of Costello (one being Annagh) and parts of another four; but since 1898 the barony has been made up of the complete civil parishes of Annagh, Aghamore, Bekan and Kilmovee, and parts of the civil parishes of Knock, Kilbeagh and Kilturra.
Our interest here, of course, is in the civil parish of Annagh, which at Ballyhaunis meets the civil parish of Bekan. The first thing that one notices about the civil Annagh on the map is that it mostly lies to the south-east of the town, so it does not take in the townland of Annagh which is beside Mannin Lake and was the site of an old castle (as we have seen), and which has always been in the northern civil parish of Aghamore, just as it was in the Aghamore-Knock Catholic parish before 1894. From these facts, perhaps a deduction could be drawn that the parish of Annagh (both Catholic and civil) did not derive it's name from the townland of Annagh beside Lake Mannin, but more likely from the other Annagh, which in the old maps is marked as the site of a church beyond Kilmullen on the other side of Ballyhaunis. But indeed, for completeness it must be said that Sir William Petty's map of 1685 shows still a third Annagh, nearer to (or even beyond) the Roscommon border.
All in all it is a question that needs more study. But in any case, in the oldest Catholic documents the name used for Annagh is 'Enagh', and this was derived from the Irish word 'Eanach' meaning 'marsh'. But the anglicised version, i.e. 'Annagh' was well established by the 17th century. Incidentally, the population of the civil parish of Annagh in 1841 was 7,904, while in 1979 it was down to 2,175.
Extract from 'St Mary's Abbey'