It may be hard for the restuarant to compensate...
Did you require them to have insurance to cover your paintings in case of fire or theft?.. If the restuarant burned down would you be expecting 8 x 2,500 or whatever the price was?
I think you may have to chalk it up to experience, .. and have a written agreement in place, and require insurance, next time you're doing something like this...
Grogans Pub apparently display artwork in a similar arrangement, maybe you could ask them about insurance, and missing paintings etc.. and damage due to fights, or spills, splashed wine etc...
At the same time I'd be suspicious that they sold the painting, and kept the money.. but I'm not sure what you can do.. so report it to the Guards, and hope for the best. If the painting comes up for sale in the future, or re-surfaces, then you may be able to claim it for yourself if you report it stolen now.
There are paintings on display in the Stephens Green Center valued at 25,000!!! and they're just sitting there on the wall!