I regularly work with copies of the parish records held in PRONI in Belfast. Whilst some are easy documents to read, many are not. The handwriting is often terrible, pages have faded, extra entries squeezed in, in the margins and at the bottom of pages etc etc. Many are damaged by water or ink or are torn. Pages are lost. And so on. It can be extremely difficult to read some entries clearly. Many RC entries are in latin. So there is no way that mistakes and misunderstandings can be eliminated from the records even if they were all computerised afresh. The only advice I can give is to use the IFHF site as a guide and a search tool, but verify everything against the original entries if you can.
Good record keeping wasn’t really much of a priority in 19th century Ireland. I looked up Drumgooland Church of Ireland records recently, and there’s a note in them saying: “The entries of baptisms from this date 2nd Jun 1833 were made from papers collected by me J. A. Bears, lying about the church and vestry room and put in this book after I received it from … Thos I. Tighe at Parsons Hill on my becoming curate of the parish 1837.” That probably tells you all you need to know about the standards of record keeping that sometimes prevailed in the 18th & 19th centuries. It was evidently haphazard at times, and consequently many early records are lost.