The reason for the location being out in the Atlantic and the radius being large is that whoever put the NOTAM into the system used the centre of the Shannon FIR as the point and a radius that encompasses the whole FIR. Remember that the FIR goes out to 15 West.
This is done where NOTAMS are for everyone who enters the FIR.
I would agree that in this case you are correct that a flight from Donegal to Sligo has absolutely no interest and furthermore, in areas such as the UK where this happens also, such irrelevant NOTAMs can hide the important ones. Thus making it less safe.
Why not ring the NOTAM office in Shannon and ask them?
Regards,
DFC