Hello :-)
The Finda Grave record for Thomas & Dorothea, indicates they married 1877.
https://www.findagrave.com/memorial/214743856/thomas-farrellFurther search leads to a Civil Record of marriage in 1877 Enniskillen, Co, Fermanagh
You can freely find and view their marriage certificate online, here:
https://www.irishgenealogy.ie/en/On 26 Nov 1877 in Derryvullan Church, EnniskillenThomas Farrell, Full Age, Bachelor, Labourer, of Derrybrusk. Father: John Farrell, Farmer
Dorothea Irvine, full age, Spinster, of Derrybrusk,. Father: Leslie Irvine, Labourer
He signed with his X. She signed her name.
Witness: Robert Farrell. Jane Irvine
Further searches ( "john Farrell" Derrybrusk) leads to:
LISNASKEA REGISTRY OFFICE MARRIAGES, COUNTY FERMANAGH
MARRIAGE 31 Jan 1853
CARSON, Joseph , Full age, Bachelor, Pensioner , of Aghnaloo, Derrybrusk Parish, Co Fermanagh.
Father James Carson, Farmer
FARRELL, Elizabeth , Full age , Spinster, of Aghnaloo, Derrybrusk Parish, Co Fermanagh.
Father: Edward Farrell, Farmer
Witness:
John Farrell & Margaret McKnightChurch: FERMANAGH, Derrybrusk Church of Ireland Marriages County Fermanagh
MARRIED 25 Dec 1883
FARRELL , Robert, age 28 , Bachelor, Farmer, of Aghnaloo, Derrybrusk Parish, Co Fermanagh.
Father: John Farrell, Farmer
KERNAGHAN , Margaret Jane , age 19, Spinster, of Drummee, Derrybrusk Parish, Co Fermanagh.
Father: William Kernaghan, Herdsman
Witness: Louis D J Kaye & Sarah D Kaye & Annie Kernaghan
The witness of the 1853 could well be the yet to be married parents of Thomas. You have put 1851 as the birth year of Thomas - could it be later?
The 1883 Marriage could well be Thomas's witness - and brother. Both the 1853 and 1883 events point to the townland of Aghnaloo, Derrybrusk for those Farrells. Knowing the townland of your ancestors is pretty key, so Aghnaloo would be a good place to start if there is nothing better? :-)
Cheers
AMBLY