You may already be aware of this, but the given names Aeneas and Eneas were Latinized forms which some priests used in baptismal registers for children who were actually name Aonghus (often anglicized as Angus). Sometimes the Latinized names stuck, but some of them, like Dionysius for Donncha (often anglicized as Denis), did not.
In case you're interested, the surname McDonnell in Irish is Mac Domhnaill, which means “son/descendant of Donal”. According to MacLysaght’s Surnames of Ireland, the McDonnell's in Connacht are descendants of a gallowglass (in Irish, gallóglach, or “foreign fighter”) mercenary family who were a branch of the Scottish Clan MacDonald. In Scottish Gaelic, the surname is spelled MacDhòmhnaill. There is no "d" sound at the end of the name in either language, and that came about in the anglicized version, McDonald, only after contact with English speakers. MacDonald is more commonly used in Scotland, but McDonnell and McDonald were both used in Ireland, often inconsistently from generation to generation. The priests who wrote down names in the parish registers, and the officials who recorded names for other purposes, were not always careful and sometimes could not hear the sounds of Irish clearly, so spelling discrepancies were common..
The Irish Mac Dómhnaill clan migrated from Scotland to the Glens of Antrim in the 13th century, and by the 15th century controlled all of North Antrim (where they can still be found), but a large number of them were brought from Antrim to what is now County Mayo in the late 14th century to assist the ruling family of that area, the Anglo-Norman Burke family. Many people in Ireland have ancestry in such gallowglass families. Although those McDonnelll's settled principally in what is now Mayo, some eventualy spread to surrounding counties, especially Galway, Roscommon, and Sligo (the county boundaries came later, anyway). I have McDonnell ancestors myself, from East Mayo. In fact, my great-great-grandfather, Patrick McDonnell, had two first cousins named Eneas McDonnell (sons of two different uncles).
