Fran,
The Glebe and Magherenan are both listed as townlands in the parish of AGHANUNSHUN in Donegal. This parish is right next to the parish of CONWAL which contains the town of Letterkenny. From what I can gather these townlands do come under the CIVIL REGISTRATION DISTRICT of LETTERKENNY. You could request a search for the birth certificate from the Registrars office at Letterkenny. The charge is 6 EUROS for a copy if found and I believe 2 EUROS if not found. I would give them a call to check postage costs to NZ. I pay 1 EURO postage to the UK. They take payts by credit card. Tel no. is (074) 9124576 or 9123772 and address is Cvil Registration,Tirconaill House, Letterkenny, Co. Donegal, Ireland. As they hold records for various registration districts in Donegal you will need to advise your request is for LETTERKENNY REGISTRATON DISTRICT.
Would be worth a try particularly as you have exact date of birth.
Angela