Irving

Irving isn't a great unit by any means but he does have enough power and bulk to kill things into late game. Unlike the rest in his class tree, he has a unique tier 2 class that really screws him over by not letting him buff with his Mastery stat in any meaningful way.

Irving is often hated on because he is - to a lot of people - a downgrade from his son. Like his daughter, I believe he is an overhated and misunderstood unit. 

Irving has a really decent Mastery stat but he doesn't need it at all. All of his skills are pretty garbage with his kit. He will be slow in any class you put him in unless you deliberately feed him Elixirs and/or the Bloodied Ring. Instead, his skills all offer a variety of QoL buffs:
Need to hit just a little harder? Use wolf to shred defense.
Need to push a foe or an ally? Shove
Need to get rid of debuffs? Use Spinning Blade
A situational Damage/Healing AoE? Champion skills.

My suggestion to Irving is to :
leave him in his base class (as it is honestly what he does best with his stats),
make him bulkier with the Dimming Ring, and either
1. go all in on your defensive capabilities with the Tranquility Ring; or
2. double down with his unique Protector passive and use the Harmony Ring to give adjacent allies +3 power. 

Other viable ring options include:
Victorious, Dauntless, Indomitable, Hallowed, Chosen, Karmic, Calamity, and Antimage. 

Summary: The key to Irving is the Dimming Ring because you aren't going to realistically get him past level 45 unless you are just absolutely babying him. With his mediocre skill options, mastery growth of 30% and gaining him so late in the game, there really isn't anything to save. Abandon mastery, embrace bulk.