AS A WRITER whose fiction was grounded in the life stories of the working-class Irish-Americans, James T. Farrell refused to dress up his work with either literary fashions or pretensions. It was as if the purity of his vocation had been linked early in his career to a driving commitment to a full description of the spiritual and emotional details of his characters. Anything less would be fakery. For Mr. Farrell, who died this week at 75, the false had no place in authentic fiction.

The results of this disciplined point of view poured forth in more than 50 books. This amazing output -- Mr. Farrell wrote every day and believed that "every mood, every passing fancy, every trivial thought can have its meaning" for a true writer -- began with the "Studs Lonigan" trilogy in the early 1930s. Read today, the Lonigan books are almost the hammers and shovels for an archeological dig into the buried history of the bleak ethnic neighborhoods of Chicago in the 1920s and 1930s. But Mr. Farrell was more than a tour guide in the slums. For one thing, the Lonigan family was anything but destitute. It was making its way upward, in the classic rise of immigrant advancement. Mr. Farrell successfully pointed out, though, that the dreariness and frustration of Studs Lonigan came from "a pervasive spiritual poverty." The institutions that should have been offering richness -- family, church and school -- were already in the 1930s breaking down.

Although Mr. Farrell never seemed to win the attention or acclaim that were given to one or another of his contemporaries -- Steinbeck, Hemingway, Wolfe, Faulkner -- his fiction had an innocence that came through in all of his prose. His style was the seeming absence of style. As one critic said in the 1950s: "Who is better -- the slide-rule fictioneer of our day, who frequently refines his short story into a weary and well-tailored void, or Jim Farrell, who reaches out with a rough hand and comes up, quite often, with an authentic fistful of human truth?"

In the pursuit of that truth, Mr. Farrell was a delightfully charming man, with a fresh Irish gaiety that he took with him on his rounds on the lecture circuit, the Manhattan salons and saloons and his beloved baseball stadiums. He was as joyous in talking about the virtues of the Chicago White Sox as those of Tolstoy, being an ardent fan of both.

Mr. Farrell leaves a treasury of novels, essays, criticism, poems and interviews. As a writer, he had the simplest of credos: nothing human is unimportant. Few American writers have been as faithful to that vision, and fewer still for as long.