There is book documenting all Boston arrivals and it contains a "John Cassaday" who was born about 1793:
Passenger Arrivals, 1819-1820. (Originally published as Letter from the Secretary of State, with a Transcript of the List of Passengers Who Arrived in the United States from the 1st of October, 1819, to the 30th of September, 1820)
With an Added Index
The unfortunate thing is that the book currently is for sale for approx. $33. I'll put a link at the end of this message in case you're interested in purchasing it.
Perhaps this will give more personal information regarding your John Cassaday. I also saw a John Cassaday (b. Clare, Ireland about 1794) enlist as a Private in the US Army in Sept. 1818 in Boston. So your John may have left Halifax more than once in that time frame.
Are you thinking that your John Cassaday went back to Ireland and stayed?http://www.genealogical.com/products/Passenger%20Arrivals%20...
Additional info - I just found this on LDS. There is a slim chance this is your John:
Massachusetts, Deaths, 1841-1915
Name: John Cassady
Event Date: 19 Jan 1867
Event Place: Roxbury, Massachusetts
Estimated Birth Year: 1793
Father's Birthplace: Ireland
Mother's Birthplace: Ireland
Volume/Page/Certificate Number: v 203 p 259
Film Number: 960191
Digital Folder Number: 4221407
Image Number: 288
This makes no mention of a wife, but does list parents, and the age is on the mark.