Hi Stephen,
I have not got very far for you. I did ring Michael Wallace, whose father was Johnny Wallace & I believe a brother of Sarah. He could not help me but did refer me to your great uncle who is in his 80's & he has no further information than what you have & he knew, when I mentioned your name, which is good as I am worried incase people think I am a scam when I ring. There are other Wallaces in Carrickacreeney, but Gt Uncle believes that they are not connected, as regards any information going back further, he was unable to help me nor with the headstone, which I will visit again for you.
He did say that he lives in the original Wallace holding, which is good as through the Valuations office http://www.valoff.ie/research.htm you should be able to track the ownership of the property. I think you should contact them & tell them who the current owner is & the previous owner would be William John & you know the property is in Carrickacreeney.
To go back further you would need to contact the Registry of Deeds http://www.landirect.ie/eng/About_Us/Registry_of_Deeds_Services/Registr…
They will tell you about the previous owners & if they are Wallaces, you will have more names to work on. In my own family memorials sometimes mention other family members & marriage settlements etc.
You could also read the transcripts from old Cavan papers http://www.irelandoldnews.com/Cavan/index.html
This would give you some insight into their lives, though it usually gives news for the very wealthy & the very poor, the ordinary classes are not covered and if your family did more than farm, they may be mentioned. You may have already written to the Church of Ireland in Shercock, if not enquire about any records relating to your family.
Will you write to your Great Uncle also as he was unsure if you got the information from them, I think they would like to hear again from you.
Best Wishes Carmel