Technically, for a turn up and go user, the time will be worse. At 15 minute frequency, the average wait time is 7.5 minutes. So you'll gain 2.5 minutes from frequency but lose 3 minutes in journey time.
People I've talked to at my station are very unhappy about this. For the normal commuter train pair on Longford commuter (0545 Sligo up, 1805 down), we are looking at 20 minutes longer in the day.