Just trying to go through the various points
The Curry/ie /(O)Neill/Rocliffe children's births are less than 100 years old so unless the mother's maiden name is recorded on the index we can't find them. The dates the index records mother's maiden name varies from place to place.
Usually when a couple are not married the children's birth are registered in the mother's name but of course a couple could claim to be married when they were not. We can't see a marriage for Curry/ie Rocliffe and the marriage we believe is Tacy and Samuel's is in the name Rocliffe while a widow would usually be in the index under her married name.
You give a birth year for Tacy of 1913 but we haven't found a birth, if the birth year is based on her age at death than it may possibly be incorrect. Death ages are often incorrect.
To find Tacy's and Samuel's birth we need at least their father's names and occupations, this information will be on the marriage cert.
The marriage cert may also clear up the issue of a marriage to Curry/ie or not.
The picking up and dropping of the O on O'Neill is normal for all names with an O and doesn't generally cause any problems.
I think what you need to do is collect as much information as you can from family, even little bit helps and get the marriage cert.
You don't need a certified one a research/photocopy will have all the same information and is much cheaper but it must be ordered by post or fax.
To order it you will need, vol, district, and year which is here
https://www.familysearch.org/ark:/61903/1:1:FYG4-2NYnormally there is a page number as well but it is missing but this shouldn't cause a problem, although it may be worth making a note of than fact on the form.
and the form on this link
https://www.irishgenealogy.ie/en/civil-records/help/i-want-to-get-a-copy-of-a-certificate-what-do-i-doFingers crossed it's the correct marriage.
We still don't know what if any connection there is to America, it could of course be on the O'Neill side.
It's great we could collect so much on the Rocliffe's in Dublin and even better to be able to trace them further back in the UK but until you know for sure who Tacy's father is it's all just possibilities.
I'll have a look at the Ervine address and see if it throws up anything.
