This is an interesting query. I'm wondering why Dorothy was Lady. That indicates she would possibly have been the daughter or the wife of a Lord or the wife of a Knight of the Realm (ie Sir). I looked at Griffith's Valuation of Ireland 1848-64. The only appropriate Ferguson with a title was in Co. Tyrone in 1851. He was Sir Robert A. Ferguson of Aghyaran, Termonamonagh. Co. Tyrone. I looked at Irish Ancestors Surname Search and found there were four families named Lilley in Fermanagh at the time of the Valuation and both Lilley and Ferguson were in the same parish in the County.
I'd suggest you post a message to Enniskillen Library as they may have something about the Ferguson family.