Well before The Flintstones, Alan Reed was well known for his work in radio, one of many radio personalities who embraced the new medium of television. In fact, virtually the entire cast of The Flintstones, especially during the first four seasons, came from radio (i.e. Mel Blanc, Bea Benaderet).
One of Alan's first television gigs was an ill-advised video version of the popular radio comedy series, Duffy's Tavern. Seems series creator Ed Gardner (Archie, the bartender) got it in his head that the show could work on TV, too, just like so many others, like Burns & Allen, Dragnet, Gunsmoke, & The Jack Benny Program. Unfortunately, Gardner made one fatal mistake. He didn't make any personal adjustments in his performance. More on this over at The Land of Whatever.
Reed, as Finnegan, shows up about 3 minutes or so into the episode, "Grand Opening".