Exactly as above. There are very few domestic flights from ICN, so the facilities in the departure area are very limited. It consists of three or four gates and one small convenience store. No lounge, no restaurants, etc. Unfortunately there's no lounge in the arrivals area either.

The domestic counter at ICN only opens two hours before departure time. So if your arrival there is earlier, you'll need to carry your bags around with you for a while. The spa/massage place in the basement will store luggage, and I'm sure there are other places that will as well.

If you do have a long layover and are still able to change things, there are many many more options GMP-CJU (or GMP-PUS, not sure which one you're flying to). The transfer ICN-GMP takes 30-40 minutes by either bus or airport train.

Let me know if you have more questions, especially if it's CJU you're flying to - I've flown SEL-CJU return more times than I care to remember.