Civil registration only takes a Catholic family back to 1864. To go further — and most families want to — you turn to the church. Catholic parish registers are, for the great majority of Irish ancestry, the oldest surviving record of ordinary people by name, and the single most important source before the state stepped in.
What the registers contain
Catholic parishes kept registers of baptisms and marriages, and sometimes burials. A baptism entry typically gives the child's name, the date, the parents including the mother's maiden name, and the sponsors or godparents — who were very often close relatives, and so a quiet map of the wider family.
A marriage entry names the couple and their witnesses. The detail is usually sparer than a civil certificate, and the Latin and the handwriting take some getting used to, but for the years before 1864 these books are frequently the only place your family is named at all.
When they begin — and why it varies so much
There is no single start date. Town and city parishes often have registers reaching back into the 1700s, while many rural parishes — especially in the poorer west — don't begin until the 1820s, 1830s or even later. Survival was a matter of local circumstance, not national rule.
This is why two families from neighbouring counties can have wildly different odds of being traced into the eighteenth century. The records that survived shape what can be proven, and an honest researcher tells you that up front rather than promising a line that the books can't support.
How to read them free online
The National Library of Ireland digitised its microfilm of Catholic parish registers and put the images online free at registers.nli.ie, covering most parishes up to about 1880. The images are browsable by parish, and several other sites have since indexed them by name to make searching faster.
Because the NLI images aren't always name-searchable on their own, the efficient method is to fix the parish first — usually from Griffith's Valuation or a civil record — and then read the register for that parish directly. Knowing the townland tells you which parish's book to open.
Getting past the handwriting
Early registers are often in Latin, with Latinised first names — Jacobus for James, Johannes for John, Maria for Mary — and abbreviations that repeat from page to page. Once you learn a parish priest's hand and shorthand, the reading speeds up dramatically.
This is patient, page-by-page work, and it rewards it. A single baptism with the right godparents can connect two branches that no index would ever have joined — which is exactly the kind of find that earns its place in a family's history.