so you are thinking she became with child at 12...
Her birthing years spans over 2 decades, maybe 3 because there is an extracted file on the IGI for a Maryanne, christened May 4 1801 Kilmuir-Easter, R&C.
As you probably know there are several submissions on the IGI as well,
with Barbara Ross as mother..so if this is the same mother, she likely was very young at marriage
starting with your Samuel in 1778 and ending with a birth in 1807...
Both of those person's dates match for Maryanne's christening and the date for Samuel's christening matched the date you have for his birth...so
this person may have researched the marriage information as well.
I think you have to order the form as I don't think they put submitter's
names & addresses on the site which is a good idea...but doesn't hurt to
find out what their source is, if you can... J.J.