Hi again janetmills,
That is a very important piece of information i.e. that Duncan is recorded in the 1841 census as born in Scotland but not in Stirlingshire! (Although this might be a trap - parts of Kippen were at times recorded as being in Perthshire.)
The more information you provide, the more likely it is that someone might be able to help you.
However, unfortunately, you might have reached a brick wall. Far from all births/baptisms or marriages are recorded on ScotlandsPeople or in the IGI. And even if Duncan's birth/baptism and/or a previous marriage are recorded on SP, how would you know you have the correct one? In the IGI, there are several births/baptisms for a Duncan KEIR (however spelled) in 1770 +/- 10 years. One of them might be your Duncan but how to tell? And there are quite a number of marriages 1800 +/- 20 years of a Duncan KEIR (however spelled); one of these might be yours but how to tell?
Have you viewed the OPR entry for the marriage of Duncan & Janet STEWART in 1820? It might say nothing more than is in the IGI but one doesn't know until one looks.
Are there any surnames used as middle names which appear in the children of Duncan & Janet or their descendants?
Were Janet and the children still in Drymen in 1851?
As well as looking for MIs in Drymen and Kippen, it might be worth a look in Buchlyvie near Kippen (in case they were of, or had changed to, the secessionist church there).
Sorry to be pessimistic.
JAP