One way is to search each census for Marys born in the IOM in 1816. I was able to do this on-line for 1881, and this was the only one of eight or so that I couldn't eliminate...she remains a possibility:
Emma JOHNSON Head W Female 35 Manchester, Lancashire, England Confectioner
Amelia JOHNSON Daur Female 11 Gorton, Lancashire, England Scholar
Annie JOHNSON Daur Female 9 Gorton, Lancashire, England Scholar
William JOHNSON Son Male 6 Gorton, Lancashire, England Scholar
Emma JOHNSON Daur Female 3 Gorton, Lancashire, England Scholar
Mary WILKINSON Mother W Female 65 Isle of Man Formerly Domestic Serv
Source Information:
Dwelling 203 Hyde Rd
Census Place Gorton, Lancashire, England
Family History Library Film 1341932
Public Records Office Reference RG11
Piece / Folio 3902 / 62
Page Number 1