Actors Burt Lancaster, left, and Kirk Douglas, right, with Mr. Shaffer at the Academy Awards in 1985. (Anonymous/AP)

Peter Shaffer, a British playwright who plumbed the recesses of the human psyche in Tony- and Academy Award-winning works, most prominently in the drama “Equus” and the Mozartean period piece “Amadeus,” died June 6 at a hospice center in County Cork, Ireland. He was 90.

The London agency of Macnaughton Lord Representation announced his death but did not disclose a cause. Mr. Shaffer had resided for many years in New York City but traveled frequently to London, where he established himself in the 1960s through an association with the National Theatre, and in Ireland, where he was visiting at the time of his death.

Mr. Shaffer, who found critical and popular favor on London’s West End and on Broadway, wrote 18 plays that encompassed tragedy and comedy, horror and farce, the epic and the quotidian, history and the present.

His writing career spanned four decades and peaked in the 1970s, when he produced his two most enduring pieces: “Equus,” an exploration of the forces that compel a stable boy to blind the horses he tends; and “Amadeus,” about the ruinous jealousy that plagued the Viennese court composer Antonio Salieri after his talent was outshone by the unhinged genius of Wolfgang Amadeus Mozart.

“Equus” premiered in London in 1973 under the direction of John Dexter and moved the next year to Broadway, where it received the Tony Award for best play with Peter Firth as the stable boy, named Alan Strang, and Anthony Hopkins as his psychiatrist, Martin Dysart.

Mr. Shaffer in his New York apartment in 1966. (AP)

The production was adapted into a 1977 film directed by Sidney Lumet and attracted Oscar nominations for Firth, who reprised his stage role; for Richard Burton, who took on the leading role of Dysart; and for Mr. Shaffer, who wrote the screenplay. Mr. Shaffer said he was inspired to write the play after hearing of an English stable boy who had committed a similarly violent act against his animals.

“In London, ‘Equus’ caused a sensation because it displayed cruelty to horses,” the Daily Telegraph quoted him as saying, “in New York, because it allegedly displayed cruelty to psychiatrists.”

“Amadeus” premiered in London in 1979 starring Paul Scofield as Salieri and Simon Callow as Mozart. It arrived on Broadway in 1980 with Ian McKellen and Tim Curry in the respective roles and received the Tony Award for best play. But it proved even more successful in its 1984 screen incarnation, which was written by Mr. Shaffer and directed by Milos Forman. It won eight Academy Awards, including best picture, best director, best actor in a leading role for F. Murray Abraham as Salieri and a best screenplay award for Mr. Shaffer.

The narrative popularized by “Amadeus” — that Salieri may have conspired to poison Mozart to rid Vienna and himself of the dissolute genius — was disputed by historians such as Robert W. Gutman, a Mozart biographer who identified illness such as rheumatic fever to be a more likely culprit in the composer’s death in 1791.

“Of course I have taken some liberties,” Mr. Shaffer once told the New York Times. “This is a play, not a biography of Mozart. But almost everything in it can be supported by historical fact.

“I am positive that Salieri intrigued against Mozart,” he said. “Who else but a jealous rival would have seen to it that ‘Figaro’ closed so promptly, that Mozart got only a pittance from the Emperor as a minor functionary, any number of things.”

He said that he was not convinced that Salieri poisoned Mozart, as was rumored at the time. “But Salieri, after all, did try to commit suicide in 1823,” Mr. Shaffer noted, “and he did claim that he had murdered Mozart. Whether he did or not, the subject was still constantly on his mind after 32 years, still haunting him. He must have had enormous guilt feelings about something.”

Peter Levin Shaffer was born in Liverpool on May 15, 1926, and grew up in London. His twin brother, Anthony Shaffer, would also become a playwright, known principally for the long-running, Tony-winning murder mystery “Sleuth.” Together they wrote several mysteries under the nom de plume Peter Anthony.

With their parents, who were Orthodox Jews, the boys moved frequently during World War II before being drafted for national service as coal miners. Mr. Shaffer later studied history at Trinity College at Cambridge, where he graduated in 1950. He lived briefly in New York before returning to London, launching his career in theater.

His first play, “The Salt Land,” about the formation of the state of Israel, aired on the BBC in 1954. His first major success came in 1958 with the premiere of “Five Finger Exercise,” a family drama originally directed by John Gielgud. It was later taken to Broadway, where it starred Jessica Tandy as an upper-crust wife, Roland Culver as her wealthy but lower-brow husband, Brian Bedford as their tormented son and Michael Bryant as the German tutor who becomes the interlocutor of their family woes.

“As an exercise in the art of expression, ‘Five Finger Exercise’ is superb,” theater critic Brooks Atkinson wrote in the Times. It is “not so much written,” he observed, “as lived.”

A 1962 film version starred Rosalind Russell, Jack Hawkins, Maximilian Schell and Richard Beymer.

Mr. Shaffer’s other early works included a pair of one-act comedies, “The Private Ear” and “The Public Eye.” The first centered on a shy man who, to his regret, calls upon a more self-assured friend to help him impress a date. The second followed an accountant and the private eye he engaged to trail his wife in search of evidence of infidelity.

Both starred the actress Maggie Smith, who later appeared in Mr. Shaffer’s play “Black Comedy,” a 1960s farce in which actors feigned groping about in the dark, and “Lettice and Lovage,” which premiered in London in 1987 and on Broadway in 1990 with Smith in a Tony-winning performance as a peculiar tour guide in a British home.

She was only one of the top-flight actors who brought Mr. Shaffer’s works to life. Another was Christopher Plummer, who appeared in “The Royal Hunt of the Sun” (1964) as Francisco Pizarro, the Spanish conquistador who defeated the Inca Empire. Plummer also appeared in the 1969 film version, but as the Incan Atahualpa, with Robert Shaw as Pizarro.

Mr. Shaffer was knighted in 2001, the year his twin brother died. Survivors include a brother, Brian. Mr. Shaffer’s works have proved enduringly relevant. In the late 2000s, a London and Broadway revival of “Equus” started Daniel Radcliffe, best known as the title character of the “Harry Potter” film franchise. Earlier, a revival of “Amadeus” featured David Suchet as Salieri and Michael Sheen as Mozart.

Throughout his life, and on his death, some observers of Mr. Shaffer’s works wondered whether the rivalries and pairings that ran throughout them — Mozart and Salieri, Strang and Dysart, Pizarro and the Incan emperor — belied a competition, latent or otherwise, between the twin Shaffer playwright brothers.

Peter Schaffer demurred, insisting that they were, simply, “enactments of my own internal tension.”