Companion active skills get actived automatically if a certain trigger is reached.
For Adaon, he can never lay traps but if he sees a trap which he can disarm with a good percentage (I think 50% at least but I'm not sure) he'll automatically go to it and try to disarm it.
For Hirge she will cast her healing spell if you or her (or your summon) have less then half hitpoints left and she still has the mana left to cast it.
I would certainly highly recommend taking the healing skill for Hirge since it's like making her a pocket healer. She can heal you if you are a warrior or your summon if you are a summoner mage or herself if you want to hide behind her. It's certainly worth it.
Ive always found that having Hirge as your companion is the most effective, with her heals of course