Noel:
All of the children from the second marriage to Susan Doherty were baptized in Moville RC church so I presume that the children born to James Brown and Fanny McCrossan were also baptized in that parish, however, the earliest Moville RC records start in November 1847 https://registers.nli.ie/parishes/0376
I checked on the subscription site Roots Ireland and did not find any baptismal records, marriage records or a death record for Fanny McCrossan. Therefore, I assume that Mary Brown was born before November 1847 to Fanny and that Fammy also died pre-November 1847.
I also did not find the marriage record for James Brown to Susan Doherty. Likely Susan came from a differnt parish and that parish does not have records back to 1848-1849 when the marriage likely occurred.
I can't answer the question regarding the Kerrigans adopting Mary Brown.
There is an August 18 1850 baptismal record in Moville RC church for a Francis Brown (not Fanny) with parents James Brown and Susan Doherty.
Here is a link to the actual parish register page https://registers.nli.ie/registers/vtls000633063#page/9/mode/1up ; and I believe I see a dot above the I so the name is Francis.
Let me know what questions you have.
Roger McDonnell
