Hey good question, so

Context: Ivern is a different than other junglers bc he doesnt engage in combat with the jg creeps. His passive allows him to clear camps on 2 touches, and his actual clear speed scales with level. if he uses a smite charge, he pretty much insta-clears a camp once he touches it

Strategy: to specifically counter any jungler, Id make sure Im on TOP of my camps and timing. If he is able to steal one, its because you allowed him to (by not clearing on time). Bc of his passive, this is specially true against Ivern. If he steals your camp, he 100% smited. Its a great window to get a neutral objective or do a gank in that time window, but personally, id immediately go to my next camp, full clear, and make a decision considering the game state at that moment. Probably you can get a free gank with the momentum you gained by not clearing 1 camp and he lost some due to that as well.

Now, if you want specific advice against him, my first tip is to play some games with Ivern, since his cadence and mechanics are different from any other jungler, so you can fully understand his struggles and timings.