has baptisms for three children of this couple:
James Smith b. 1751 Dreghorn
Andrew Smith b. 1753 Dreghorn
Archibald Smith b. 1762 Kilmaurs
Given the dates they probably had more children who either didn't survive or their baptism record didn't survive.
The same web site has several births for Elizabeth Orr in Ayrshire, with several in Kilburnie. The best site for Scottish BMD, however, is http://www.scotlandspeople.gov.uk
which is a pay to use site.
Not being able to find a birth or marriage in the OPRs for that time isn't unusual - not all records survived, the registers were't always well kept, and not everyone had their children baptised. Irregular marriage was also common as it was legally binding in Scotland but wasn't recorded.
The locations point to a coal mining family, and such families often moved at regular intervals. There were coal mines in the Parishes of Kilmaurs, Dreghorn and Dundonald in the 1700s.