It is bounded on three sides by rivers, (Shannon, Little Brosna and Lusmagh, or Rapemills, River), so it is almost an island.
Lusmagh is succinctly described in the following stanzas of a poem by Peter Kenny:
In Lusmagh lies but herb and plain;
No mountains high does it contain,
No valley deep, nor balmy beach
Is near to sooth the mind of siege.
But we have woodlands and the bog,
To soothe and swab the soul of sweat,
To sweep the cobwebs and the fog
That troubles, toil and cares beget.
Across the Shannon from here is County Galway, and Lusmagh was once part of County Galway. In church organisation it continues to be part of the Diocese of Clonfert, a Galway diocese.
To the south, across the Little Brosna river lies County Tipperary. Victoria Lock, an aid to navigation of the Shannon, at the south west corner of Lusmagh, is beside the point where the three counties (Offaly, Galway and Tipperary) and three provinces (Leinster, Connaught and Munster) meet.
An ancient name: The word "Lusmagh" means "Herb (lus) Plain (magh)," and is very ancient. It is said that the name was given to the area by the ancient Dana People (Tuatha Dé Danaan).