Perhaps you've already thought of trying the following, but I thought I'd ask...
Would you know James' occupation, please (around 1855)? Perhaps he may be found in a city directory in either Boston or somewhere in New York? Was there ever any mention of siblings of either James or Jane?* Was Jane her actual name or could it have been a nickname? Did they follow the Irish naming pattern, which could lead to the names of their parents or possibly siblings?
*I've been researching on and off for many years (first starting while I was in college). I thought an ancestor, for whatever reason, was an only child. With the help of RootsChatters and someone from another site, it was determined that she had several sisters and possibly at least one brother (who died when young). While I've never found her Irish grandparents, it did help with other records and details.

PS It has been mentioned previously in other non-RootsChat threads about McKeown/Mills, but in case you've not seen this record:
New York Passenger List
Caledonia arriving 16 June 1848 from Londonderry, James McKeown, age 23.
Of course, there's nothing to show that it isn't another James.
