This is a movie with a considerable reputation--mostly bad. Some of that lack of regard is appropriately earned. However, there are good aspects to Parnell, too. It is definitely worth reconsideration after over 80 years since its initial release.
Was Clark Gable miscast? It is indisputable that there were better choices to play the inspiring, charismatic Irish champion whose great dream was for home rule for his country. But Gable was certainly capable of playing such a decent, noble character with honest conviction. He did so with considerable sincerity in The Misfits. And even though Gable made him seem more American than Irish, we should remember that Parnell's mother was a Yankee, and he always had a close connection with the United States.
Was the film misdirected? John M. Stahl was not in his usual element in tackling such a dense biographical/historical drama. But on the whole, Stahl obtained compelling performances from his actors, and the narrative moved at a pace that held the viewer's attention. MGM showered him with a stellar cast of players and its usual glossy production values. The end result is both entertaining and interesting.
Did the film's historical inaccuracies contribute to its lack of success? Other commentators have pointed out these flaws, and they need no repeating here. However, the biographical film genre by its very nature is often full of contrived fiction usually inserted to make such movies more likely to be commercially acceptable. Is Parnell any worse than MGM's Boys Town, Young Tom Edison/Edison The Man or Madame Curie In this regard? I think not.
In the end, Parnell (the movie) was probably doomed because Gable's fans could not accept him playing an obviously non-Gable part---much like what happened to Tyrone Power when he did Nightmare Alley, Cary Grant when he starred in None But The Lonely Heart, Spencer Tracy when he took on Dr. Jekyll And Mr. Hyde and Robert Young when he appeared in They Won't Believe Me. Gable had a specific carefully created star image and was usually cast in roles that burnished and enhanced that image. Playing Charles Stewart Parnell--an almost God-like idealist, leader and patriot--definitely went beyond being cast against type. His fans were obviously disappointed, and the movie accordingly failed at the box office. Perhaps much of this result was caused by the unpleasant surprise of his fans seeing Gable trying to do such a role, rather than due to intrinsic faults in the film itself.
Take another look at Parnell and judge for yourself.