Apologies for the long delay in replying.
As you say, Michael and Margaret were married c.1794 on the 19-May-1793 to be exact, in the parish of Castletown, Co. Laois. Unfortunately neither of their parents' names are recorded on their marriage certificate so this is probably as far as we can go with their history (unless there is a Canadian death record or grave info that records something new). Witnesses to the marriages were Garret Kavanagh and Mary Kelly.
You can see the original here:
I looked for baptisms for children of this couple but there were none listed in Loais or anywhere in Ireland. I also checked for baptisms for any Michael Cassin in and around the time prior to 1770 (assuming he was at least 20 when he married) but there are no Michael Cassin records prior to 1810.
RC baptisms for Castletown begin in 1772 so he must have been born prior to this and therefore baptised before that time making him older than 20 when he married.
I checked how many Cassin listings there were for children baptised in Laois and there are 165 matches in all but only one that takes placed in or around 1770 (the period around which Michael was probably born). This record dated 01-Aug-1769 is for the parish of Rosenallis and records the baptism of a Hyam Cassin. Sponsors names are Michael Moare and Jane Thomson so I feel this is a different family.
You can see where the parishes are on this map link:
Let me know what you think. The NLI facility to see the digitised copy of the original is a great resource