Thanks for the tips.
It was a Church of Ireland marriage- I checked that the minister was C of I. I believe it was a civil record- found it thru FHL microfilm. They were all farmers.
What I've found so far: John in a Poor rate valuation report online. http://eppi.dippam.ac.uk/documents/11536/eppi_pages/268223
There is also a John Wallace in Griffiths- different parish- Emlaghfad- so I'm not sure if it's the same man.
The description of the Sir Robt Gore Booth estate papers at PRONI mention leases with different Wallaces at Cullaghbeg, who could be related. I hope to view these papers eventually if I make it up to Belfast.
I plan to visit the Rep Church Body Library to view parish records. Not sure what to look for when visiting Sligo itself- graveyards I suppose.