Mild spoilers ahead. A royal visit to Downton Abbey is the basis for all the drama in the first feature film based on the series, including plenty of disagreements between the Crawleys' servants and the royal staff. But there's a more serious incident that's also a part of the plot. At one point, there is an assassination attempt on King George V, who is staying at Downton along with a few members of his family.
Fans of the show know that the estate itself is fictional. As the movie points out, Downton is meant to exist in Yorkshire, but the village nearby is also not real. And while the Earl of Grantham was a real peerage title long ago, it's been out of use for centuries. Downton Abbey is based in reality, however. Families the Crawleys did exist, and sometimes, real people and real historical events are weaved into the story. The very first episode found the family losing an heir in the sinking of the Titanic, and now, the movie brings real monarchs to visit.
The visit that King George (Simon Jones), Queen Mary of Teck (Geraldine James), their daughter, Princess Mary (Kate Phillips), and her husband, Henry Lascelles (Andrew Havill) make to Downton in the film is inspired, in part, by a real visit to Yorkshire the King and Queen took in 1912, as series creator Julian Fellowes explained to Vanity Fair. For their part, the Crawleys have to host a lunch, a dinner, and a parade. It's during the parade that the assassination attempt on the king takes place.
At the start of the movie, Captain Chetwode (Stephen Campbell Moore), comes to town with a mysterious and great interest in Tom Branson (Allen Leech). Branson believes that the man is suspicious of him because he's an Irish republican. Instead, Chetwode is the one who is against the monarchy and intends to kill King George with Branson's help. When Branson realizes what it happening, he decides to take matters into his own hands, tackling Chetwode to the ground before he can shoot.
Of course, this is where the movie's plot departs pretty significantly from reality. There are no official reports of an assassination attempt on George V. (But, in the movie, it's kept secret, so maybe there's something we don't know...) There is this blog post with information about a potential failed plan to assassinate the King in Belfast that didn't come to fruition. There was also a bombing in Belfast in 1937 that could have killed killed King George VI, George V's son.
The Downton Abbey movie is set in 1927. The Irish Free State was formed in 1922, but conflicts continued over Northern Ireland, which was still under the rule of Great Britain. But the film doesn't delve very deep into the politics of the time.
Based on the lack of solid information about an assassination attempt on George V, it seems that this plot line isn't meant to refer back to history. Instead, the mention of Irish independence is meant to create some drama that is true to Branson's character. The former chauffeur has had to choose between his political beliefs and the Crawleys many times. He might not be a monarchist, but he's unwavering in his support for his family. Anyway, no matter his beliefs about Ireland, he's never been a murderer.