Hi Dano,
I had a look to see if I could find a birth for Agnes McCluckie/McLuckie 1719 +/- 5 years - no luck.
I looked to see if Alexander had any siblings and it seems he (possibly) had three. There is a 12 year gap between John and Hellen and also Hellen seems to have been born in Larbert rather than St. Ninians but the two places aren't very far apart so that wouldn't count her out.
LAIRD, Robert
Gender: Male
Christening Date: 15 Nov 1707
Recorded in: St. Ninians, Stirlingshire, Scotland
Father: Robert LAIRD
Mother: Margaret COWAN
Source: FHL Film 1041956 Dates: 1688 - 1819
LAIRD, Alexander
Gender: Male
Christening Date: 28 Mar 1710
Recorded in: St. Ninians, Stirlingshire, Scotland
Father: Robert LAIRD
Mother: Margaret COWAN
Source: FHL Film 1041956 Dates: 1688 - 1819
LAIRD, John
Gender: Male
Christening Date: 18 Sep 1712
Recorded in: St. Ninians, Stirlingshire, Scotland
Father: Robert LAIRD
Mother: Margaret COWAN
Source: FHL Film 1041956 Dates: 1688 - 1819
LAIRD, Hellen
Gender: Female
Christening Date: 4 Oct 1730
Recorded in: Larbert, Stirlingshire, Scotland
Father: Robert LAIRD
Mother: Margret COWAN
Source: FHL Film 1041953 Dates: 1723 - 1855
I would imagine that the best you'll get on Alexander & Agnes would be a 'Banns' listing from the OPRs rather than an actual marriage, although I couldn't find anything on the BVRs.
Sorry,
Anne