You should be fine. There are some assumptions here. You can get your bag checked all the way to NRT and your flight arrives at ICN on time. If this is your situation, the transfer takes only minutes. As you deplane your first flight, head to the transfer area and present new boarding pass. 10-15 minutes.
If you are not able to check your bags to NRT, then this is where the fun begins. I've done this in under 30 minutes but it can take up to an hour moving at a leisurely pace. Assuming your flight arrives on time, you are one of the first ones off the plane, have already filled out the immigration paperwork or using APEC card, your luggage has priority tags, filled out the customs declaration, then it should go smoothly. Then once you've gone upstairs and rechecked bags for the next flight, get right back into the security line as that gets busy in the mornings. APEC card will also help you here, you can go to the side for priority screening.