has three choices for a David Millar marrying a Catherine Marshall. Only one of them spells the name "Millar":
16 DEC 1825Dalmeny, West Lothian, Scotland
The other two spell it "Miller" and have them married in Edinburgh, Midlothian. I expect you'll go through your life telling people "Millar with an 'ar' at the end". If you continue, don't overlook the possibility that some clerk heard "Millar" but wrote "Miller".
If you pull up the "Millar" IGI citation, click on "Batch Number" and enter just the surname "Millar" you'll get 107 people in that batch with that surname. The citations don't say how they are related, however.
If those aren't the right people, you may want to post again with some dates and places, if you know them.