In 1911, songwriter Ernest R. Ball is dismissed from his job as an instructor at the Cleveland Conservatory of Music for writing sentimental ballads. Ernest then goes to a burlesque theater in search of Belle La Tour, a singer to whom he has written regarding one of his songs. In Belle's dressing room is Mary "Irish" O'Neill, a diminutive singer and dancer, who is not afraid to let loose with a dangerous left hook when confronted by a fresh man. Irish, who is in the dressing room while recuperating from an encounter with the overly familiar stage electrician, does not reveal her true identity to Ernest and instead encourages him when he sings one of his ballads. Belle arrives and spoils Irish's deception, however, by firing her and ordering Ernest to leave. Outside, Ernest and Irish talk as they stroll and discover that they are both orphans who have dreams of building successful careers in New York City. Having fallen in love with Irish, Ernest begs her to postpone her trip to New York until he has enough money to accompany her, but she insists on leaving the next morning. Determined to raise the cash for the trip, Ernest goes to a vaudeville house to sell one of his songs. Middleweight champion boxer Stanley Ketchel is moved to tears by one of Ernest's touching ballads, but his cantankerous manager, Edgar Brawley, assumes that Ernest stole the song. Ernest angrily leaves, but returns during Stanley's show, when Stanley offers twenty-five dollars per round to any man who can fight him. The sentimental Stanley refrains from pounding Ernest too hard, and the young composer wins seventy-five dollars, with which he then travels to New York. Despite his best efforts, Ernest cannot find Irish and soon takes a job as a song plugger with Leo Betz's music publishing company. Leo sends Ernest to a fancy nightclub with a tune for vaudeville singer Lucille Lacey, but when Edgar, who is surprised to see the young man, laughs at him, Ernest launches into a well-received rendition of one of his own compositions. Lucille is so impressed with Ernest that she takes him under her wing and introduces his songs in her shows. In less than a year, Ernest becomes well-known and his songs are performed throughout the country. Although Lucille is interested in Ernest romantically, he still thinks only of Irish and is delighted to find her working as a hat check girl at a posh restaurant. Irish is embarrassed by her lowly job, however, and refuses to believe that Ernest still loves her, but he finally persuades her to quit her job and leave with him. While Ernest is waiting for Irish, he overhears Edgar making a bet with Lucille's smooth-talking friend, Al Jackson, who always wins their wagers. Edgar bets $25,000 that he can make a musical comedy star out of the next woman to leave the ladies lounge, and although Al tries to cheat, Ernest arranges for Irish to be the woman Edgar must pick. The confused Irish believes that Ernest and his high society friends are making fun of her and storms out, and Al insists that Edgar stick to his bet. Hoping to get rid of Irish so that she can have Ernest to herself, Lucille arranges for her to be employed in a small theater out of town. Although Ernest and Edgar conduct a frantic search for Irish, Al finds her first and promises her a job at his new theater in Cuba. As Irish prepares to sail to Cuba, Ernest finds her and pleads with her to return to him. Mistakenly believing that Ernest loves Lucille, Irish refuses, and the dispirited Ernest disembarks. After the boat sets sail, however, a drunken Al reveals the true nature of the bet to Irish, and that Ernest really does love her. Irish returns to New York aboard the pilot's boat, but when she goes to Ernest's hotel, she discovers that he has left abruptly. Edgar comforts the heartbroken Irish by putting on a new show with her as the star. He advertises the use of Ernest's new song, "When Irish Eyes Are Smiling," and Ernest, believing that Edgar has stolen his composition, rushes back to New York. Ernest and Irish are reunited backstage, where she tells him that she will give him a black eye unless he kisses her. The show is a success, and although Edgar happily collects his wager from Al, who has returned from Cuba, he is infuriated to learn that Al is the ultimate winner because he is the show's secret backer.