Place names often refer to townlands, which are the smallest unit of land division in Ireland. A database of these can be searched at -
www.thecore.com/seanruad I dont see any close matches on the townland database for Killonahen,
Given the context I would have said that it maybe an electoral district, but there is a civil parish with the name -
Killonaghan Probably the same place name written phonetically in the records you viewed.
The civil parish contains about 18 townlands, you can search for these on the townland database.
Shane