I have had a look at your posting tonight and decided to have a look at the records.
I dont know if you knew already that John Sullivan and Mary Shanahan had 9 Children together and not 8 as you mentioned. I thought at first that they had 10 children but then found that one of them, Johanna was listed twice. John & Mary were married on 27 Jan 1853. As you had said, she also had 4 children with Pat Donovan. Both families had lived at Burgatia, Rosscarbery..You can see the full listing of the Sullivan side of the family on the following link.
I can send you the other family link if you want it..The following link is to the baptisms of all Mary Shanahan's in the Rosscarbery area from 1815 - 1825. These are the only dates which I feel twould put Mary at the correct age span to have the number of children she had and be the legal age to get married. From the names which recur in her family, and from my experience, I think that the Mary Shanahan born in 1820 has the best parents names to make them the match those repeated in her later family. I also have found a few possible siblings of Mary but will await your opinion on what is in this post before i follow them up further.
Let me know what you think as soon as possible so that I might have the best chance of following through on this. You may contact me on my personal email address if you wish. it is, firstname.lastname@example.org