Ference dumped the puck into the Rangers' end and then skated hard to New York's goal-line, where he shoved McDonagh from behind at 1:50 of the extra period.
"Before either player could touch the puck, Ference, coming with great speed, makes contact with McDonagh's right skate and then shoves the defenceless McDonagh on his numbers, causing McDonagh to impact the boards violently," NHL disciplinarian Brendan Shanahan in a statement post on the league's website Sunday.
Ference was given a five-minute major penalty and a game misconduct for charging by on-ice officials. The Rangers scored the winning goal on the ensuing power play.
"While we would agree with Ference's assertion that there was no malicious intent, this nevertheless is a reckless shove from behind that causes a dangerous collision with the boards," Shanahan added. "It is a textbook example of the boarding rule that was rewritten for this season to include pushes or shoves and reflects emphasis on impact with the boards."
Shanahan also noted that McDonagh had done nothing to contribute to the hit.
Ference will forfeit US$36,486.48 in salary under the terms of the NHL's collective bargaining agreement. The money goes to the Players' Emergency Assistance Fund.