On the topic of mirror image and shield combo, lets say a creature successfully attacks the real one and then the defender decides to use shield as a reaction, which comes first and which has priority?
Mirror image is resolved before shield can be cast.
Shield requires that you be hit by an attack, having a casting time of 1 Reaction:
which you take when you are hit by an attack or targeted by the magic missile spell
Mirror image is triggered before you are hit with an attack:
Each time a creature targets you with an attack during the spell's duration, roll a d20 to determine whether the attack instead targets one of your duplicates.
So mirror image is always resolved before the trigger for shield can even happen.