Continuity mistake: During the extended fight scene, Will Danaher and Sean Thornton go crashing into a hay stack. As they emerge, they announce to the crowd that they are going to fight by the Marquis de Queensbury Rules. As they make the announcement, Sean Thornton's back is covered in hay. As he turns, the camera cuts to follow him, and there is instantly no hay on his back.
Continuity mistake: Barry Fitzgerald meets with Maureen O'Hara as the shocklehorn (matchmaker). When they go into the house, it's a clear day. As they talk in the house, every time the camera turns to O'Hara, the window behind her shows a rainy day, but every other view out every other window shows a clear day.