Ancestral records for Irish Presbyterians do not, typically, date much further back than the 1820s. Which is not to say there are NO really old registers. The earliest surviving register is for the parish of Antrim in County Antrim and dates from 1674.
That is a rarity, however. Few go back even to the 18th century, let alone the 17th. The reality is that by far the largest proportion of Presbyterian registers don't start until the 19th century.
And just because one register from a particular congregation starts in one year there's no guarantee that all its documents start in that year.
You don't have to look far to find quite major differences.
Presbyterian registers are similar to those of other denominations in that they were not standardised in format or content, there are sometimes huge gaps in the years recorded even in a single congregation, and inconsistency means there may be significant changes in the information recorded even by the same minister.
Also, legibility either due to the passing of time, the level of care taken to preserve these valuable ancestral records, or the minister's standard of handwriting can be an issue, especially with the earliest registers.
The contents of baptism records differ from parish to parish. Some contain only the child's name, the father's full name and the mother's first name. Some go a little further, especially later registers, to record the parents' residence, the mother's maiden name and the name of godparents (sponsors).
The date of birth may also be given, especially if it was some time before the baptism. This is usually the case when a Presbyterian family had 'saved up' their children, probably to save money on the Minister's fee, for a multiple baptism ceremony.
Presbyterian marriages sometimes provide very detailed ancestral records, but you might have to hunt for them as the ceremonies were often conducted in secret.
The earliest registers usually provide only the names of the couple and the father of the bride, while later ones – certainly from 1845 – provide the names and residences of the bride and groom, their marital status, ages and occupations, and the names and occupations of their fathers.
notice of Presbyterian weddings had to be sent to the Kirk Session. These
notices were then recorded in the Session Minute Book. These Books survive, so
it's worth checking them for ancestral records.
Burial registers are uncommon as few churches had burial grounds.
The Public Record Office of Northern Ireland (PRONI) has microfilmed nearly all registers of Ireland's Presbyterian congregations. According to the Presbyterian Historical Society (see below), PRONI's 'coverage is so extensive that there are relatively few congregations whose records have not been deposited in some form in PRONI. So, clearly, PRONI should be your first stop.
The Presbyterian Historical Society (PHSI) has copies of all of the microfilms of Presbyterian registers held by PRONI and these can be viewed at the Societys Library and Archive in Assembly Buildings, Fisherwick Place, Belfast ,BT1 6DW, Northern Ireland. While the Society can provide genealogical advice and guidance, it does not have a full research service.
In addition to the PRONI microfilms, a small number of Presbyterian records are only available at the PHSI office. A list of these registers and a guide to the Society's collection can be found at their website www.presbyterianhistoryireland.com. These include some baptism and marriage registers and some very early Session Minute Books.
A small number of pre-1900 registers and other records are still held by the local Minister.
Local genealogy centres: Transcriptions of some registers may be online at RootsIreland.ie. Check the online sources menu for the relevant county to see if the congregation you're interested in is included in the database.