Season 6, Episode 18, "Lauren"
Through flashbacks, we learned that Emily went undercover as “Lauren” while dating Doyle. She had moved in with him, played an active role in his young son’s life, and was with them at their Italian home when Interpol arrested him. While imprisoned in North Korea, Ian was shown photos of his son's alleged murder and immediately went on a rampage to track down all of the parties involved with his arrest.
While being tortured, however, Emily admitted that Ian's son, Declan, was alive and well. She had faked his murder to allow him a new start, away from his evil father.