Nova Scotia Supreme Court Judge Nick Scaravelli sentenced Falconer on Tuesday to life in prison with no chance of parole for 25 years.
Kirwan was last seen alive by friends after leaving the Dooly's pool hall in downtown New Glasgow in the early hours of Oct. 9, 2011. Her remains were found about a month later in a shallow grave in Heathbell in Pictou County.
The 12 members of the jury in Pictou reached their verdict on Tuesday after 7.5 hours of deliberation.
The trial lasted for close to three weeks.
Falconer, 31, had pleaded not guilty to first-degree murder.
The Crown presented witness testimony and text messages that traced Falconer's movements from the time Kirwan was last seen. Her body and clothes were found in a shallow grave close to the home of Falconer's stepsister and there was a bloody tank top found in his car, with both his and Kirwan's DNA on it.
2nd murder conviction for Falconer
Falconer had pleaded guilty to second-degree murder in 1998 and was sentenced to life in prison. He and another teenager killed cab driver Robert Leblanc, then went joyriding in his taxi.
The killing happened on Heathbell Road, the same road where Kirwan's body was discovered.
Falconer was released after serving more than 10 years in prison for Leblanc's murder.
Police suspected Falconer early in their investigation of Kirwan's disappearance. That suspicion was enough to have Falconer's parole revoked in February 2012. He was returned to prison until he was formally charged three months later with Kirwan's murder.