Joyce modeled "Mr Deasy" on Francis Irwin, the owner and headmaster of the Clifton School in Dalkey, where he worked for several weeks or months in March-June 1904. Altering some details and keeping others, he created a vivid portrait of a blustery old man whose authoritative advice is undermined by frequent errors of fact.
Deasy is richly characterized by a host of little physical details that make him appear antiquated and decrepit: his "gaitered feet, the "angry white moustache" that is also called a "rare moustache" as he blows the wispy threads away from his mouth, "the honey of his ill-dyed head," the "coughball of laughter [that] leaped from his throat dragging after it a rattling chain of phlegm." Preparing to utter wisdom, "He raised his forefinger and beat the air oldly before his voice spoke." These unattractive physical attributes cohere with his habit of not listening to what other people are saying, and his curmudgeonly insistence on giving unwanted advice.
According to Ellmann, Irwin was "an Ulster Scot, very pro-British" (153), and this conservative Protestant heritage is strongly pronounced in Deasy. Gifford adds that the fictional name "may owe something to the Deasy Act (1860), an act ostensibly intended for land reform in Ireland but in practice a ruthless regulation of land tenancy in favor of landlords (i.e., in favor of the pro-English, anti-Catholic Establishment)" (33). Deasy regards the Catholic, nationalist, freethinking, and wildly prodigal Stephen as an antagonist: "I like to break a lance with you, old as I am." Stephen might well say of him what he says in Circe about the British soldiers: "I seem to annoy them. Green rag to a bull." In addition to his Protestant respect for money, which Stephen might learn something from, he represents many values that cannot benefit his employee: Unionism, anti-Semitism, and misogyny.
Ellmann observes that Francis Irwin was an alcoholic with a very red nose, and that his malady forced him to close down the school not long after Joyce's departure. Joyce left out this detail, and gave Garrett Deasy (we learn his first name in Aeolus) an estranged wife which the bachelor Irwin did not have. He also incorporated characteristics of Henry Blackwood Price, a man he knew in Trieste: "preoccupation with a distinguished Ulster ancestry," and "interest in the hoof-and-mouth disease" (153).