Borresaine does not give the stun effect (-30 Deflection, -30 Reflex, -4 Perception, -4 Intellect, -4 Dexterity, Allows Rogue Sneak Attack) when you make a crical hit and hit with the effect. Instead the enemy can't take any action or move.
Borresaine does not renew it's effect if you hit with the stun effect while the enemy allready is allready stuned by the bow. If you score the stun effect while, the stun effect gets a new time, but there is no effect on the enemy for the time of the new stun.