One of my great-grandmothers grew up just a couple of townlands east of Derrynagower, so I can offer some additional info.
You can see more information about the townland of Derrynagower (including an interactive map) at this link:
https://www.townlands.ie/mayo/clanmorris/mayo/kilcolman/derrynagower/
There are links at that site to other info about the townland, including a link to the record for the townland in Griffith's valuation (mid-1800's), where a John Melia is listed as a tenant in the townland, as you can see at this link:
https://www.askaboutireland.ie/griffith-valuation/index.xml?action=doNa…
Since he's the only Melia listed, he was likely the father of your Patrick Mealia.
Mealia and Melia are two ways that the North Connacht Irish surname Ó Máille was anglicized. It was perhaps most often anglicized as the well-known name O'Malley (less often as O'Mailey), but the Mealia and Melia forms are not uncommon in Mayo.
As you can see from the first link above, Derrynagower is located in the civil parish of Mayo, but the local Catholic parish is called Mayo Abbey. Civil parishes evolved into administrative districts, and do not always have the same boundaries (or even the same names) as the more modern Catholic parishes (which are often larger, encompassing some or all of several civil parishes). Comparing the available maps, though, Derrynagower does appear to be in the Catholic parish of Mayo Abbey. The parish records for Mayo Abbey can be viewed online at this link, and you can download copies of any relevant pages there:
https://registers.nli.ie/parishes/1076
The records go back to 1841 for marriages and baptisms, and earlier for some marriages, so you may be able to find baptismal records for Catherine and her father, and perhaps even a marriage record for John Melia, who may be Catherine's grandfather. That should give you her grandmother's name as well. Take with a grain of salt any year or age references you have, because people in those days were often not certain when they were born. If you do not have any success, you could try the parish records for Kilcolman, the Catholic parish just to the east (where my great-grandmother was baptized), because the boundary between the two parishes looks to be very close to Derrynagower. I have sometimes found relatives baptized in adjoining parishes, and you can access the records for that parish through the map at the last link I gave you.
When the counties were created (at a fairly late stage in many places), the county was often named after a major town in the area, and County Mayo's name was taken from Mayo Abbey.
