Martha Desmond

Martha and her uncle Richard were the only surviving members of the Wall family who died on the voyage from Ireland to Grosse Isle just above Quebec City. Most of the Desmond family survived (less Catherine and Margaret died at sea in 1847) and Richard & Martha were cared for by the Desmonds and later both married members of this family.

Both Pat and Martha had "Marrah" as place of birth when married.as opposed to a County in Ireland or Ireland.- place name unknown - but a guess is Murragh parish in Cork which is adjacent to Brandon parish from where Pat's parents originated.
This is correct. A record was found Murragh parish Co Cork on Jan 31, 1843, for the baptism of Catherine of John Desmond and Ellen Hickey. https://registers.nli.ie/registers/vtls000634891#page/68/mode/1up

Martha Wall was Richard Wall's niece.
https://www.familysearch.org/library/books/viewer/673795/?offset=0#...

Immigrated in 1847 (1901 Census).