The details for the David Forsythe 1848-1933 on Find a Grave clearly state that he is the son of David Rankin Forsythe and so not the David born in Ireland whose father was William, therefore the one who is in Carlisle all his life seems likely to be the David who was born in Ireland and the marriage certificate would clarify this. I just hoped that someone might just have had some paper records for St.James Church in Carlisle that would have given a quick answer.
I think the replies that have come in today from Sandra and trish1120 confirm my theory but I'll send for the marriage cert just to make sure. Many thanks indeed for all the information.