Townland names sometimes changed over time, and there does not appear to be a townland in Aghagower/Aughagower now by that name. There is also the possibility that it was what is known as a "sub-townand", a name used by locals for an area within an official townland. Sub-townland names did sometimes appear in older records as townland names.
There are three townlands in Mayo, all based on the Irish name An Gabhlán ("the fork", like a fork in a creek), which have been anglicized as Gowlaun or Goulaun. Two are in East Mayo, in the neighboring parishes of Kilmovee and Kilbeagh, and the third is in northern Mayo, in the parish of Crossmolina. You can see all of them in the list of Mayo townlands at this link: https://www.townlands.ie/mayo/
Sorry not to be able to help further there but, in case you are interested, I can provide some inffo about the Gavan surname. There is an Irish surname originating in north Connacht, Ó Gáibhtheacháin, which was anglicized in several ways, such as Gavaghan, Gaughan, and Gavan (the Gaughan surname also exists in Ulster, but is based on a different Irish name).