IMHO, limousine buses are typically the way to go to/from the airport, being they're reliable, comfortable, and low-cost in comparison to a taxi. However, I don't recommend it for someone as yourself, being you're arriving in Korea by another means, then trying to catch one, without knowing where the bus stops are. In your case, I think just taking a taxi is best.

But if you really don't want to spend the money for a taxi, have a look at the following which lists the various limousine buses with routes to ICN:
http://www.airport.kr/airport/traffi...=1200970530809

If you can make your way to a bus stop on one of those routes and wait around for the next bus, it'll almost certainly be doable for a fraction of the cost of a taxi the whole way. If nobody here has any further information, I'd imagine there's some sort of information booth at the pier when you arrive that you could ask for directions to the nearest limousine bus stop, and have them write it down in Korean to give to a taxi driver to drop you off there.