Someone I know (Not selling it) has a 5 gallon cask of Jameson which was a Vinters Association offer about 50 years ago. Unmolested and with labels for the particular pub of the time.
Would the whiskey still be drinkable or would that depend on storage?
Also - assuming circa 25 bottles left in it (is this reasonable?) - what would it be worth today?
The simplest thing here would be to get on to one of the Irish whisky auctioneers and make an enquiry. I did this with a vintage bottle of Tullamore Dew a while ago. And if you were considering selling it (if it's intact) then that's the way you might wish to go anyway.