Hello,
she is listed as born Llanelly in 1871.
There are no obvious births but what is strange is that there is an Abby Brian (o) with the same details in 1871 census.
She is entered with her widowed father James at 20 Oxon Street whilst the Collins family are at 24 Oxon Street.
There is also a 5 year gap between Abby Collins and the next Collins child - unless there were births and deaths?
Unfortunately, I can't find a birth for Abby O'Brian either

I am having difficulty finding them in 1881 also.

1861 has th O'Brian parents born in Waterford so no family connection but mother Margaret dies in 4th quarter 1870so I suppose there could be a possibility that Abby O'Brian was staying with them and just got misnamed in the census.
regards
heywood