Civil Registration Office, Co Donegal By far my preferred way of obtaining certificates/copies. All queries and payment can be handled over the telephone, while you wait.
Address: Registrar of Births, Deaths and Marriage, Tirconaill House, Letterkenny, Co. Donegal, Ireland
Telephone: +353 749124576 or 353 749123772
Opening Hours: 09.15 - 16.30 M-F Late opening Thurs 19.30

Or, if you want to try buying online see below. Be sure you have all the info before you order certificate.
http://www.rootsireland.ie/

Frank