Hi there,
Statutory registration was no introduced in Scotland until 1855, so there are no certificates for before this date. At this point you are into church records. If the Dewars were Church of Scotland, then the details you are looking for would be in the Old Parish Registers. You can search these online at
www.scotlandspeople.gov.uk but you can't view the images. I would check earlier posts in this section for 'Where to start in Scottish Research?" as more information will be given there, along with what to expect from the records.
Anyway, I had a look for births of a Mary Dewar to a parent named David. There only seems to be one Mary Ann Dewar showing up, born (or batpised) on 15th December 1823 at St Cuthbert's parish, Edinburgh, but the parent's given are David Dewar and Margaret Plenderleith. I couldn't see any children around the time frame with a mother called Mary Ann Creasey.
I also looked for marriages and did not find one for David Dewar and Mary Ann Creasey.
I hope this will help a little.
Cheers,
Clare