Irene Adler was an adventuress in the late Victorian period. Born in Trenton, New Jersey, and ostensibly occupied in the profession of opera singer (in which role she, was, it should be noted, famous), she became the mistress of the "King of Bohemia". When she tried to blackmail him with comprising photographs, the King employed Sherlock Holmes to recover them. But Irene outwitted Holmes, as described by Dr. John Watson and Arthur Conan Doyle in "A Scandal in Bohemia", and forever afterward he admired as the only woman to have done so - "The Woman!"
It is of considerable debate amongst students of the canon as to the extent of Holmes's admiration for Irene, whether it was purely intellectual or perhaps romantic/sexual in nature.
The Sherlockian scholar John D. Clark, in refuting Bernard DeVoto's theory that Mycroft Holmes could be the father of Nero Wolfe, published an article "Some Notes Relating to a Preliminary Investigation into the Paternity of Nero Wolfe" in the Baker Street Journal in 1956 that proposed that Holmes reincountered Irene in Montenegro in 1891, after faking his death following his momentous battle with Professor Moriarty in "The Final Problem". Holmes was in disguise, and as he was no longer a detective his usual objections to romantic love could not be held to apply, and so, as the theory goes, he and Irene embarked upon a relationship that culminated in the birth of Nero Wolfe. William S. Baring-Gould developed Clark's theories in his books Sherlock Holmes of Baker Street and Nero Wolfe of West 35th Street, and concluded that Wolfe's best friend Marko Vukcic was also born as a result of the 1891 encounter, and that Irene later remarried and gave birth to Wolfe's nephew (Wolfe's mother is also mentioned as still being alive in some of the novels by Rex Stout).
Earlier, in 1950, another Sherlockian scholar, James Montgomery, had announced that he had found evidence in his family papers that "Irene Adler" was merely the stage name of his aunt, Clara Stephens. He produced various documents in support of the claim, and further material was released by Baring-Gould in his works on Sherlock Holmes.
These theories cannot be said to be universally-accepted. However, it must be noted that Philip José Farmer, in Tarzan Alive, accepts the balance of evidence that Holmes and Irene were Wolfe's biological parents.