I should have looked up ancestryireland and the 1851 census , and familysearch.org but jumped in
1823-1837 Tithes Applotment Books see Irish Genealogy Hub
here is your Thomas Crossans below at Bahernamaghery ie Barnamaghery, Killinchy.
1823-1837
a single alphabetical list of each name entry,
containing the townland, & year recorded,
with the addition of parish and county
based upon the work of the
Irish Genealogy Hub’s
listing for each parish in Northern Ireland,
which are quicker to lookup if you know the parish!
The relevant original books are in the
Public Record Office of Northern Ireland (PRONI) Belfast
Killyleagh-Down
Crossan, James-Townland: Ballywoolen Year: 1827-Killyleagh-Down-
Crossen, Andrew-Townland: Cluntagh Year: 1827-Killyleagh-Down-
Crossen, David-Townland: Cluntagh Year: 1827-Killyleagh-Down-
Crossen, John-Townland: Ballymoolen Year: 1827 Notes: John and Samuel-Killyleagh-Down-
Crossen, John-Townland: Ballywoolen Year: 1827-Killyleagh-Down-
Crossen, Samuel-Townland: Ballymoolen Year: 1827 Notes: John and Samuel-Killyleagh-Down-
Crossen, Samuel-Townland: Ballywoolen Year: 1827-Killyleagh-Down-
Simpson, Robert-Townland: Tullyveery Year: 1827-Killyleagh-Down
Killinchy-Down-
Crossans, George-Townland: Bahernamaghery Year: 1833-Killinchy-Down-
Crossans, Thomas-Townland: Bahernamaghery Year: 1833-Killinchy-Down-Simpson, James-Townland: Ballygowan Year: 1833-Killinchy-Down Crossen, Jackson-
so you may find James or Robert Simpson is the father of your Mary
from
http://freepages.genealogy.rootsweb.ancestry.com/~rosdavies/SURNAMES/C/CrocCrot.htm#cror its easy to abstract all the Killinchy and Killyleagh heres a starter
Miss Agnes CROSSEN . Killinchy wife of William Price ; mother of Margaret Jane b. 10 Feb 1868 & bpt. Raffrey Presbyterian Church (regd. Killyleagh
Miss Mary CROSSEN . Killinchy wife of Richard McBride ; mother of Elizabeth b. 17 Jun 1855 & bpt. Raffrey Presbyterian Church PR
Miss Mary CROSSEN/ CROSKERY . Killinchy wife of George McJury ; mother of William Henry b. 6 Jul 1858 & Ellen b. 20 Jan 1860 & Martha b. 22 Oct 1866 & bpt. Raffrey Presbyterian Church
Finally church records Ros davies guide gives two parishs Raffrey records start1843
http://freepages.genealogy.rootsweb.ancestry.com/~rosdavies/PHOTOSwords/KillinchyAll.htmhttp://freepages.genealogy.rootsweb.ancestry.com/~rosdavies/CHURCHES/K_W.htm#Killyleagh from the Ros Davies listings it appears the family was Presbyterian but which church did they attend as the growth was on from 1670 in the area with Presbyterian Non Subscribing Church in Balloo townland, on the Killinchy to Saintfield road it looks like you have a good chance of finding that marriage ie dont give Good luck Jim