I'm subscribed to the Rangers calendar via GCal and they just updated it with the round one information.
All season long, if a game was either on ESPN or TNT, they would indicate it in the calendar invite, otherwise it's assumed that all games are on MSG. For the first round of the playoffs, It's noted that ESPN has the first game but there's no information for the other games which leads me to believe they are all on MSG regionally.
If you're not local, I'm not sure of the alternatives.