Shaz14, there is indeed a Montrath in County Westmeath, not to be confused with Mountrath in County Laois. Montrath in County Westmeath is a townland of just under 1,000 acres in the civil parish of Rahugh.
Many Irish placenames are repeated in many counties, even the same placename can appear in several locations within the same county. Because Irish placenames are generally from the original Irish language and are descriptive of the geography or topography of their location. they're often not just found once or in just one county.