EXCLUSIVE

Guinness, the famous Irish stout, is being sued because THE BEER IS ACTUALLY MADE IN CANADA!

Kieran O'Hara is a local beer drinker from Massachusetts now gunning for Guinness, claiming the beer maker is duping millions of beer enthusiasts.

According to the docs -- obtained by TMZ -- O'Hara claims Guinness plasters a misleading label on every bottle of Guinness Extra Stout that makes people think it's a 100% Irish beer -- with ONLY a teeny tiny disclosure on the back of the bottle saying it's ACTUALLY made in Canada.

O'Hara isn't just in it for himself -- he's trying to put together a class of other Guinness drinkers who thought they were drinking an authentic Irish beer.

We reached out to Guinness for comment ... so far, no word back.