In "Cyclops," the citizen sits tucked away in the corner of Barney Kiernan's pub with his dog, Garryowen, drinking one beer after another. The citizen is the double for the Cyclops Polyphemus from the Odyssey, except that here being "one-eyed" is not a literal but a figurative trait.
Almost from the instant the reader sees him, it becomes apparent that the citizen is a narrow-minded Irish nationalist. He speaks to his dog in Irish; he thinks that the revival of the Irish tongue is inevitable; he goes on at length about the strength of the nation and the weakness of England; he curses anyone that thinks otherwise. The citizen's nationalism is the worst brand, bordering on primitive tribalism. For him, it is Ireland versus the world. Everything connected with Ireland is exempt from criticism, and everything that is not is worth disparaging. His comments quickly reveal themselves as essentially bigoted and xenophobic. In the myopic and mean-spirited nature of his thoughts, the citizen comes across as essentially "one-eyed."
It is worth noting that, though the citizen does not come off well in the episode, his views were not at all extreme in Dublin in 1904. This can be seen by the fact that the other characters gravitate toward him and let him dominate the conversation with his forceful opinions. Generally speaking, the citizen is representative of an Irish attitude that was not uncommon in 1904. Strongly (sometimes violently) resistant to English oppression, the attitude was fiercely nationalistic and insular, constantly willing to proclaim the greatness of the Irish people and Irish culture, but impervious to influence from outside the island.
The citizen is prejudiced against Bloom from the start. The very first thing he does is complain about Bloom's pacing out front of the pub. The citizen pegs him as "a freemason" (12.87). Later, when Bloom begins to disagree with some of the citizen's nationalistic sentiments, the dislike moves toward downright animosity. The citizen begins making digs at Bloom's Jewishness, and when Bloom finally speaks up, the citizen seizes the opportunity to mock him. He is particularly disgusted with Bloom's simple assertion that he is Irish because he was born there. For the citizen, nationalism can't be explained so simply (or accurately). For him, it is all-encompassing. Belonging to a nation is much like belonging to a gang.
When Bloom leaves, the citizen insults him excessively. He doubts Bloom's ability to father children, and when he hears that Molly miscarried, he suggests that it undermines Bloom's masculinity. Throughout the episode, Bloom never once makes a personal attack on the citizen. He disagrees with his narrow-minded ideas and pushes for moderation, but the citizen turns the argument into one about Bloom himself – his ethnicity in particular. When Bloom returns and the citizen continues to mock him, Bloom starts yelling back at the bigot. The citizen is infuriated by Bloom's (true) assertion that Christ was a Jew, and looks for something to throw at him. The tin falls short, and as it clinks in the street it seems to symbolize how pathetic the citizen's discrimination is in the first place.
Gifford's annotations suggest that the citizen is modeled on a man named Michael Cusack. Cusack founded the Gaelic Athletic Association, a group so focused on Irish pride that they actually denounced people as un-Irish if they watched English games like football (soccer) or rugby. What is more interesting, though, is that Joyce's biographer, Richard Ellmann, suggests that the character is loosely based on Joyce himself. Some of the short-sighted nationalist sentiments expressed by the citizen come straight out of pieces written by Joyce at a younger age. We here see the author reflecting on himself, critiquing himself, having a laugh at his own expense.