I've read the biography on which this movie is based, and found that the screen adaptation hasn't embellished the true story (unlike most biopics) very much. "Nora" covers the Joyces courtship in Dublin, immigration, and their years in Trieste until 1914 when "Dubliners" was finally published. McGregor may not be perfectly cast - he's heavier, and not much of a tenor - Joyce, on the other hand, was so renowned for his singing that he briefly contemplated making a career out of it. Ewan's very earnest in his attempt, though, he gets an A for effort. Susan Lynch makes a great Nora, no complaints there.
Some of the chronology is played around with - the Prezioso episode happened after James and Nora's last mutual visit to Galway, not before. Two of Joyce's sisters are conflated into one, and the family's stint in Rome is omitted. Some episodes are invented for the sake of exposition, such as the run-in with the cattle in Dublin. Those are minor quibbles. The film got many details right - not the least of which was how elegantly the couple always tried to dress despite their relative lack of funds. Trieste hasn't changed much since early 20th century, so we also get to see some nice location shots - and a few in Dublin.
I was a bit disappointed that the film ended at an early cutoff point, after James and Nora's first decade together. Ideally, it should have been a miniseries a la "John Adams", and covered their fame in Paris, Nora and the kids caught in a Civil War crossfire during their last trip to Ireland, Joyce's eye troubles, Lucia's illness, etc. As is, it shows only the beginning of an almost 40-year relationship.