The only definite information we have on the McDonagh family is that they came from Co. Sligo. However in his book "Ancestral Quest, Tracing my Ancestors in Ireland" (2008), Patrick Lavin provides us with the following information on the Sligo origins of Mary and her husband, Laurence O'Hara:
"Laurence and Mary often went back to visit their families in Sligo. Those visits were usually for a short period, possibly only a day or two. When Laurence went alone, he went on horseback; when Mary accompanied him, they traveled by horse-drawn vehicle of some kind. The direction he took was along the Ballintogher road.....that would seem to suggest that the destination was some place between the line Lough Arrow/Riverstown and the Leitrim border.....the length of the visit, mode of transport and the poor quality of secondary roads at the time would have their destination at not more than 15-20 miles away. Therefore, it can be accepted, with some degree of certainty, that the area in County Sligo where Laurence and Mary came from was somewhere within the Kilmactranny-Riverstown-Ballintogher triangle".
It's not known when exactly Mary and Laurence, settled in the Cootehall area of Co. Roscommon, but by 1833, they were well established in Cootehall, with Laurence and his sons farming in the townlands of Clegna, Knockadaff and Glooria.
Note: The McDonagh surname (also spelled McDonough) is common throughout Ireland, especially in Connacht. It's derived from the Irish "Mac Donnchadha" from the Irish first name Donnchadh meaning "brown warrior".