I assume that you definitely know that the John/Mary Mahon who married in 1841 are the same folks as the James and Mary Mann who had the 7 children?
Thus said, have you seen the references on the Tasmanian archives - permission to marry for John and Mary in 1840 indicates Mary is free. If you cannot get to the archives, they have a very reasonably priced research service and would check for any information on Mary and her family/arrival. I don't know of any online indexes for free settlers into Tasmania.
There is also a Mary Mahon listed as arriving in 1833, who is tried in Tasmania in 1837. Slightly confusing as to whether or not she arrived as a convict.