The fly-half, a member of England's Six Nations Championship squad, has agreed terms that will keep him at Irish until 2010. It is the 20-year-old's first senior professional contract.
Geraghty said: "I joined the London Irish academy two years ago because I thought it was the right place for me, and everything that has happened to me since has proved that was a good decision.
"I am surrounded by very talented players, and I am confident that it is only a matter of time before we win a major trophy."
Exiles rugby director Brian Smith added: "Shane is a wonderful talent, and he has made the most of his opportunities playing beside Mike Catt and Seilala Mapusua at London Irish."