Hi Lisa,
I looked up the British Vital Records but could'nt find any births matching your couple Alexander Yorkstone and Ann Donaldson.
There only seems to be one birth in Larbert around 1846 +/- 5 years and that's the following:
YORKSTONE, Alexander
Gender: Male
Birth Date: 24 Sep 1844
Birthplace: Larbert, Stirling, Scotland
Recorded in: Larbert, Stirlingshire, Scotland
Father: Alexander YORKSTONE
Mother: Ann KAY
Source: FHL Film 1041953 Dates: 1723 - 1855
I looked for any births for Alexander Donaldson 1819 (again +/- 5 years, basing Alexander's age being 20 when he married Ann) and found this - if he was born in Larbert?
YORKSTON, Alexander
Gender: Male
Christening Date: 20 Jul 1817
Recorded in: Larbert, Stirlingshire, Scotland
Father: John YORKSTON
Mother: Elizabeth ESPLEN
Source: FHL Film 1041953 Dates: 1723 - 1855
Also, I tried to find Ann Donaldson. There were 2 born in 1819 +/- 5 years, one in Kilsyth and one in Dunipace. I would think the one in Dunipace is the more likely candidate as Dunipace and Larbert are almost side by side. Here's her details:
DONALDSON, Ann
Gender: Female
Birth Date: 24 Mar 1819
Christening Date: 24 Apr 1819
Recorded in: Dunipace, Stirlingshire, Scotland
Father: John DONALDSON
Mother: Janet MALCOM
Source: FHL Film 1041941 Dates: 1708 - 1856
There is one Yorkstone MI (from pre-1855 MI's, East Stirlingshire) in Larbert Old Parish Churchyard.
The entry reads: "1835; Alex Yorkstone & Ann Kincaid"
The stones in this churchyard were transcribed in July 1969, so whether or not the stone is still there or if it's still legible, who knows.
Get back to me if you'd like me to go and have a look, Larbert is only 10 minutes from where I live.
Anne