Are you saying Anne, Mary and John are sibs? ..."Ann or Anna and sister Mary came to the USA in the late 1840's"
She landed in Mississippi? Anna MAHAN was born Abt. 1829 in Killooley, Kings County Ireland451, 452, and died 14 Nov 1875 in Natchez, Adams County, Mississippi453. She married James Edward FAGAN on 06 Aug 1854 in Natchez, Adams County, MS.
http://www.rootsireland.ie/http://www.rootsireland.ie/ - 3 candidates +/- 5 yrs: Church Baptism Mohan Anne 1828 Co. Offaly Church Baptism Mahon Anne 1833 Co. Offaly Church Baptism Mohan Anne 1824 Co. Offaly
$5EU to view the record ... but you'd have difficulty confirming without her parents names on a US doc ...