The place Nergena is mentioned in Kernogan's Kilrea, page 1.
Linde Lunney says of this name:
I've had a look through what materials I have on placenames, and can find no information which contains the name "Nergena". It is most likely that it is a misreading of some Gaelic Irish word or an attempt by an English clerk to record the pronunciation of a name which he heard as "Nergena". It is possible that the initial "N", not a common initial letter in Irish names, represents the Irish particle "na" which means "of" or "of the". According to a book on the local placenames, there is a form "Erginagh" of the modern townland name of Erganagh in Kilrea parish, so I suggest that "Nergena" may represent (Bally) na Erginagh; another 1609 form was apparently Ballinekerguey.
However, there is a townland Erginah (Erganagh or Arginah) about 2kM north west of Kilrea, and the names are too similar, so I suspect the name Nergena is in fact this townland!