Teester
Looking at http://maps.openstreetmap.ie/os_opendata.html?zoom=15&lat=53.13514&lon=-6.10645&layers=B0TTTTTFFFF, the two Bellevue Demesnes are split by the Kilcoole/Delgany barony, CP and ED boundary.  

OSi Opendata suggests that they are separate townlands (as evidenced by the different reference numbers in the pink OSi overlay).  GSGS3096 also has two separate Bellevue Demesnes, though they are somewhat hard to discern as the smaller one is so narrow.