From mirror image:
"A creature is unaffected by this spell if it can't see, if it relies on senses other than sight, such as blindsight, or if it can perceive illusions as false, as with truesight."

Intellect Devourer has blindsight and is unaffected by mirror image.

Also, devour intellect is a saving throw and not an attack so mirror image wouldn't work anyway.

Your best best is probably to find an headband of intellect which will change your saving throw from -1 to +4. However, it will take an attunement slot and you only have 3.

Saving throws are one reason you would want to get paladin to level 6 since it helps offset your low saves.