River to Donegal Bay (4)
I believe the answer is:
'river to donegal bay' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for erne that I've seen before include "Sea bird: lough in Ulster" , "A high-flier" , "Irish lough" , "'Sea-eagle, river and lough (4)'" , "White-tailed eagle" .)