One certificate, one office
Document legalisation for Ireland has been a one-stage job since 1999, when the country joined the Hague Apostille Convention. An apostille issued in Reading for use in Ireland stands on its own, and nothing further is arranged behind it. The route is so short that most of the wasted effort on it comes from misreading the certificate rather than from missing a step.
It does not vouch for the words
The check performed is narrow and mechanical. An officer compares the signature or seal on your sheet with a specimen already held. Nothing in that act touches the content. People sometimes send a document hoping the stage will confirm a date or a spelling for them, and it never does; a correction has to come from the body that issued the record.
It does not change the language
The certificate is attached to your paper, not written into it, so the sheet arrives saying exactly what it said before. Treating the two as one purchase is a common way to be surprised later. An apostille from the United Kingdom to Ireland covers the origin question and stops cleanly at that boundary, leaving anything about wording as separate work.
It does not decide anything for you
The last limit is the one that matters on a deadline. A certified file is a file that can be trusted to be what it claims, which is not the same as a file that gets the answer you want. Build your timing around the reader at the far end, and treat the single certificate as the easy part it genuinely is.