Irwin was last seen in Upland, California on September 13, 1999. He has never been heard from again. He vanished a month after signing over his financial assets, worth about $250,000, to two women, Marcia Ann Johnson and Judy Gellert. Irwin also signed an agreement to turn his estate over to them when he died. Johnson reported him missing and told police Irwin took a train to Seattle, Washington. After his disappearance, she emptied his bank accounts, sold his house, and burned down his cabin for the insurance money.

In 2001, Johnson confessed to shooting Irwin and dismembering his body because, she said, he exposed himself to her. She later recanted the confession, but was convicted of murder, burglary, grand theft and insurance fraud in November 2004 and sentenced to life in prison. Gellert pleaded guilty to receiving stolen property and testified against Johnson at her trial.

Irwin's remains have never been found.