I was at GRONI today and the guy on the desk told me that if you have the year, quarter, district, volume and page details (available from the familyseach pilot site) you can order a "transcription" of birth and death registration entries from them by phone, paying by credit or debit card. This will cost £6, rather than the usual £12 for a certificate. For marriages its best to phone the local registration office (i.e. the local council covering the area where the marriage took place) - they can accept payment over the phone and send you a transcription of the marriage entry. Most councils I've tried charge around £3 for this service.
This compares with (I believe) €6 for a photocopy of the entry from Roscommon, but there you have to pay in Euros or send your credit card info (incl security code) by post which some Rootschatters are reluctant to do. [Please someone let me know if this RoI information isn't right as I have't used them recently so can't speak from personal experience.]