From IGI:
Neil MCKAY m Catharine STEWART, 30 Dec 1844, Logierait
Parents Neil MCKAY & Catherine STEWART:
John MCKAY, b 19 Oct 1845
Neil MCKAY, b 25 Sep 1847
Both Little Dunkeld
As Catherine & little Neil are with her father in 1851, perhaps Neil snr had died?
JAP
PS: Gadget, I didn't know that you had looked at Christina's death cert. I thought it was her son John's 1855 birth cert which you looked at which led you to Charles STEWART & Christian SCRIMGEOUR, and to her birthplace.
I'm not really thinking anything in particular except that if might even have been a grandmother's name - or, of course, just have been a total error.