It's very common to find the same placename used more than once in Ireland. There are 4 different townlands all called Brannockstown, as well as a parish by the same name according to this site:
1. is in the parish of Brannockstown in Co Kildare 2. is in the parish of Gilltown, also in Co Kildare 3. is in the parish of Trim, in Co Meath 4. is also known as Belford and is in the parish of Enniscoffey, Co Westmeath.
If you have no idea which your ancestor may have come from, I suggest you search Griffiths to see which the name Brogan appears in, and then focus your research accordingly. Mostly people gave their townland as their address, but occasionally they used the parish, so you would need to bear that in mind when searching. Griffiths allows you to search by both townland (using the placename option) as well as by parish, so it’s easy enough.