The Irish Family History Foundation undertook the task of transcribing all of the surviving church/civil B/M/D rcds. At present it has done this for 26 of Ireland's 32 counties. Occasionally it adds more rcds to those already transcribed. This is sporadic and cannot be relied upon to significantly increase the size of the database.
To my knowledge, the RC church at least has retained ownership of the originals. I have noticed that some posters to Genforum have managed to get copies of church B/M/D registrations which AREN'T in the IFHF database. Whether these copies were made from originals which no longer exist, are illegible, or weren't loaned by the churches to the IFHF, I don't know. Again, this is very rare.
Civil B/M/D registration began for protestants in 1845. It was extended to the entire population in 1864. They were originally kept in the civil parishes. Late in the 19th century, Dublin decreed that these should be shipped and kept at the Four Courts building in Dublin. Some counties sent all the records, some sent only copies and retained the originals. In any event, the Four Courts building was seized by the IRA in the 1922 civil war. The provisional government used force to drive them out. Whether by IRA or governmet stupidity, the building was set on fire. Most of the records were destroyed along with much of the early censuses from 1821 up to 1891 along with 1,000 years of recorded Irish history. The British added to the chaos by destroying several decades worth of the censuses in 1916 in a "War Economy" move.
The GRO and PRONI have cobbled together whatever civil rcds which could be found in the counties. Remember, these only date from 1845 for the 12% of the population which was protestant and 1864 for the 88% who were RC. To add to the difficulty, many sects continued to keep church B/M/D rcds in parallel. In some cases, hard-shell religious members refused to participate in civil registration. It took a number of years and hefty fines before civil registration became the norm.
Starting in the mid 19th century and especially in the 1916-1923 era, rioters and government repression resulted in reciprocal burning of the other side's churches. Hence, surviving protestant rcds are very sparse at any date, except for Londonderry and Dublin. In these towns, there are a few rcds which date back to the 17th century; these are very rare.
Sorry to write at such length. This is the result of years stumbling through the self inflicted ashes of Irish history. In short - don't expect too much from either the GRO or PRONI. Surviving Presbyterian rcds can most likely be found by contacting the church synod.