I'm almost there.
Sorry if this is boring people, but I may as well post it here in case its of use to anyone in the future.
Reply from GRO Ireland, to my question about if they send out scans of original, written at the time the event was registered at the local registrars, or scans of a 'copy' of those registers.
The lady at the GRO explained that what I got was a scan of a 'copy' of the original register.
Just as in England & Wales, you went to the local registrar to register your child's birth. He wrote it in the register and, I assume, would have supplied a certificate to say he had done so.
At the end of the quarter/year, whatever the specified period was, he would copy the entire original register for that period and send it off to the GRO.
Local registrar, did and still does, retain the original register book.
She didn't specify how GRO Ireland would have compiled the index they have (which gives the GRO ref to apply for a copy cert) but I think I am safe in assuming it was the same system as was used in England & Wales. GRO gets all the copy books from all over the country, an army of clerks lists all the names registered, gives each of them a GRO ref which is different from the entry number in the local book, then sorts them by year/quarter, then alphabetically and compiles the index. Done in handwriting in the earlier years, then later by typewriters and these days, I assume, in a computer database.
LOTS of room for errors, both at local level copying the registers and then at GRO level copying names, adding in central refs and sorting them into alpha order. On the whole it all worked quite well though.
She said the year of birth (1883) on the scanned copy is correct as per her copy register and she had also checked with the registrar in Tipperary who confirmed the same entry in the original register.
Tipperary Family History Research, at a hefty charge - but it is what it is and I want the info, apparently don't have parish registers after 1878 but have transcribed and checked them all. There IS a corresponding baptism for this child, 2 days after her birth in 1883, parents correct as per the civil cert. There is 'nothing' showing in their records for a baptism for a child of this name/ these parents in 1889.
So far so good, the cert issued in 1917 for a birth year of 1889 is looking increasingly likely to be an error on the part of the local registrar.
My final step is to ask the GRO what, if anything, is in the copy register for 1889 for the date on the cert issued in 1917 and if they can check it with the local registrar. If there IS an entry in 1889 for this child, its not in the GRO index but if they can find one I will get the ref and purchase the scan. I suspect they won't find one though and at that stage I'll be done.
I emailed the GRO today to ask, gave a link to a scan of the cert we have with the 1889 date on so they can 'see' my problem. Will wait till they get back to me :-)
Boo