The marriage between Patrick Farnen and Elizabeth M’Donnell was registered in Lisburn 1868, Volume 16, page 878. If in doubt as to what is on it, you can order a photocopy from GRO Roscommon for 4 euros. I’d suggest that Elizabeth’s father was named Terence not Ference. There’s a death for a Terence McDonell, est year of birth 1821, registered in Lisburn 1877, Volume 16, page 445.
I wouldn’t be too concerned about the variations in spelling. That was very common at that time. A wish to have an exact spelling is very much a 20th century phenomenon. Prior to that no-one bothered that much, and when folk weren’t that literate anyway it often varied according to the whim of the official recording the information. Likewise ages vary considerably in the censuses all the time.
It was common for some surnames to become “Scotified” when people moved there to live. So McDonnell in Ireland often became McDonald in Scotland. Rainey in Ireland became Rennie inn Scotland etc,. There are plenty like that.