Thanks to JP08 on Belfast Forum I got the following suggestion
"The danger is always present that we apply modern intrepretations to old words, acts and customs.
The word mechanic goes back to the 17th century and it was used in at least one sense which relates to manual labour. It could be applied to anyone that had a trade or craft, like an artisan and perhaps even a cobbler."
I checked out the dictionary which gives, among the usual modern meanings, the following alternative definitions:-
"handicraftsman";
"a term of contempt for the lower orders";
and, an archaic meaning for 'mechanical', "manually employed".
I'd be interested if anyone else has found this occupation on certificates in the same period to see if the term was in general use.