Captain Boycott is remembered for not what he did or wore or said. He is recalled for what was done to him because of his behavior to others. In 1878 the lower classes in Ireland began a series of acts of violence against landlords, especially Protestant landlords. The reasons were connected with unfairly high rents and misuse of economic power to force people to do what their landlords wanted them to do. The worst incidents resulted in the murders of landlords (Lord Leitrim in 1878, Lord Mountmorres in 1880) and Mr. Walter Bourke (an agent for a landlord) in 1882. Boycott was a rather heavy handed landowner, who threw out tenants who did not do what he wanted (including voting for Tory candidates). He became targeted for punishment, but what happened turned out to be more effective and less bloody than what happened to Leitrim, Mountmorres, and Bourke.
On the advice of cooler heads than armed terrorists, such as political leader Charles Stewart Parnell, it was advised to socially and emotionally ostracize Boycott. Instead of threatening his possession of his estate (which would allow him to call in British troops) the locals would not sell him needed food or supplies, nor do work on his estate, nor even talk to him and his staff. This ostracism (now called "boycotting") proved very effective - it led to Boycott, after nearly half a year's struggle, to give up his estate and leave Ireland forever.
The movie basically tells the story, with Cecil Parker actually playing the central role of Captain Boycott. Parker plays one of his rare negative characters, but he has moments of typical befuddled Parker humor (when talking to a friend about the unwanted London reporters in his home following the ostracism campaign, he notes the reporter for the TIMES appears to be drunk under his dinner table - we see the man's legs).
Since he is the villain actually (with Mervyn Johns as his sneaky estate agent ably assisting him), we look elsewhere for our hero. Here it is Stewart Granger, as an independent minded tenant of Boycott's, whose prize race horse excites the Captain's greed. But Granger is also a voice of reason. Although he doesn't care for Parnell (he mentions rumors about Parnell's sex life), he hears the great man give a speech advocating peaceful protest rather than violence. This is how the campaign against Boycott begins.
Alistair Sim plays the local Catholic pastor, who also counsels peaceful protest. Noel Purcell plays an interesting semi-villain - the local schoolteacher who is an accomplished agitator (for violence). And playing Parnell for one scene only is Robert Donat, who gives the great man's speech his normal eloquence, and actually looks like Parnell (unlike Clark Gable in the 1939 biographical fiasco from Hollywood). A little stiff at times, it is (on the whole) a wonderful example of a good historical movie. As such I recommend catching it when you can.