Grace/Gaynor children baptism records? What are the odds?

Hello!

One of my great great grandmothers emigrated to the U.S. in 1854 under the name Mary Grace Melville (she was about 15 or 16 years old).  Her sister Mary Ann (Fannie) Melville emigrated a short time before her.  Despite the surname differences, they both claimed in various documents that Martin Grace and Margaret Gaynor Grace were their parents and that Teresa Grace and Julia Grace were their sisters (all of whom emigrated to the U.S. within the next decade or so).  There is also evidence of another sister who has not yet been identified.

At rootsireland.ie I found baptism records for Teresa, Julia, and Catherine Grace born to Martin Grace and Margaret Gaynor in Roscrea.  The baptismal information for Teresa indicates a birth year of 1846 rather than the 1852 date estimated from U.S. census documents and the death certificate. The baptismal information for Julia indicates a birth year of 1844 rather than the 1848 date estimated from U.S. census documents and the death certificate. 

The only other piece of information that I have about origins is Martin Grace’s death notice, which claimed that he “was a native of County Limerick.”

Does anyone have an opinion as to how likely it is that the baptism records from Roscrea are those of the ancestors I am researching (or are those names so common and/or the birth year discrepancies so large that they could be another family with identical names)? 

I really don't have a sense for the odds in a situation like this, so I thought I would ask the experts!  (And if you have any advice on how I might discern my great great grandmother's true relationship to Martin and Margaret, that would great, too!).

Many thanks in advance - 

John