The round-robin game was the first between the teams since Canada's 3-2 overtime victory in the Olympic gold-medal game in February in Sochi.
Irwin tied it at with 4:57 left, and Johnston added her second power-play goal of the game with 3:40 to go. Johnston tipped Courtney Birchard shot from the point past goalie Alex Rigsby with Hilary Knight serving a penalty for roughing.
Dani Cameranesi opened the scoring for the Americans early in the first, and Johnston tied it midway through the period. Shiann Darkangelo gave the United States the lead on a power play early in the second.
Genevieve Lacasse made 30 saves for Canada, and Rigsby stopped 18 shots for the United States.
Earlier Wednesday, Finland beat Sweden 1-0. In the opening games Tuesday night, the United States beat Finland 5-0, and Canada topped Sweden 2-0.