The online manual doesn't seem to have details but if you have a cheap multimeter around, you can make some educated guesses.
The larger diameter pin is probably for headphones, tip of plug to one earpiece, middle ring to the other earpiece, ring closest to plug is ground.
The smaller diameter pin is probably the microphone, tip of plug to the mic, middle ring push to talk and the ring closest the plug is ground.
Connecting a multimeter on the ohms setting between ground and the rings of the larger plug should generate a scratching noise or a click in the earpiece as contact is made between the base ring and each of the middle and tip rings. A resistance should show on the meter between 8 and 32 ohms.
The small plug should show high resistance between the middle ring and the base ring. This should drop to near zero when the push to talk switch is pressed. If this doesn't happen, try between the tip and base. The other ring is then the connection to the mic.
This is pure guesswork, but based on experience with other Icom products.
If none of this makes sense, then you probably shouldn't be mucking about with it in the first place! :-)