There's a lot going on in this thread so bear with me if I miss something.
First, and this is key, there are several very successful variations for Hirge as a companion. It's really about what suits your particular needs and playing style. If you're having fun and are able to progress through the game, than awesome, the build works for you.
With that in mind, let's discuss Hirge.
There are two basic aspects of a build:
1) How does the build do damage?
2) How does the build survive damage?
For Hirge that means asking yourself a few question:
1) 2-H vs 1-H? For Clerics, occasionally using a light hammer as needed (fire damage for trolls etc) is expected, but there's not really a long term benefit to building an agility Cleric (i.e. light weapon) be it Hirge or the Main Player. This question also directly answers if you're going to be using shields for Hirge.
2) Might or no Might. A maxed out Might skill provides impressive per hit damage (+16 physical damage, +4 fire damage). You'll feel the difference. Down side is that Hirge's AI likes to use Might against everything, including fire elemental that get healed by fire damage as well as Level 1 goblins.
3) Duel or no Duel. For 1-H Hirge, maxed out Duel is almost a requirement. Duel goes along way in adding the required damage to be viable in the mid-to-late game. Clerics, as a class, tend to be pretty durable, but they tend to suffer in the damage department. Duel fixes that. If you're playing with a 2-H Hirge, Duel is still useful in longer fights, but you might not see as much dramatic difference as you would with a 1-H Hirge getting Duel.
4) Heavy Handed or no Heavy Handed. Keep in mind that this skill requires STR 4, so if you go this route, you'll need to invest in STR, which you likely would do anyways. Heavy Handed provides a nice per hit damage boost. I won't say it's required, but really does add a notable edge to attacking.
FOR TAKING DAMAGE
1) Shield use. You answered this question when you decided if you wanted to use 2-H or 1-H weapons. Shields for clerics can add some much needed armor in the early game, and be a source of elemental resistance in the mid-to-late game.
2) Heal, Intervention, or both. Giving Hirge Heal will add a lot of durability to her as a Companion. Plus her overflow will heal you (when she remembers to heal you). In contrast, giving her maxed Intervention will allow you to use the Immortal buiild system. Each 35 mana from a mana potion concerts into 280 hitpoints. That's hard to beat. Downside is you'll use a lot of mana potions and every time Intervention triggers, it uses all your mana, which doesn't leave much for other skills.
1) Hirge's other skills can be useful, but aren't as vital to a build.
Crusader: I personally like Crusader for the extra death damage and small stun chance. It's really useful in the Forbidden Pit and against strong undead. However, against most enemies, it's a lot of skill points for what is arguably a small stun chance ( I like it, but it's more of a personal choice than a mathematical argument).
Nivaria's Barrier: If you have skill points to spare, not a bad place to put them but you won't notice to much if how Hirge plays in a dungeon with them. (Note if you're going full undead killer build, than yeah, use Barrier.
A favor towards STR over AGL seems to best. Companions get less trait points than the main character, as such, it's harder to raise more than one trait to a middle value. For Hirge, that tends to mean getting STR 4 first. That extra damage is notable in the early game and allows getting Heavy Handed, if you want it. There's nothing quite like opening up Hirge's advanced skills and realizing you'll have to grind for 3 levels to be able to get them due to trait restrictions.
Keep in mind having Hirge have an EVEN amount of AGL will increase her attack with 1-H weapons.
PERS 2 allows Hirge to learn Duel, and PERS 3 or 4 gives a nice mana pool for her, but more than that is not really viable given the limited trait points to spend.
Hirge is best as a tank I think, because she can heal herself. Don't get intervention unless you are late game, as she can respawn with half health. Griss does much better damage I have found, and griss can also do aoe damage. I don't see Hirge being used much in Ironman except as a tank. Adaon can handle traps which makes him more ideal for already tanky characters. After Hirge does her companion quest you get access to more fighter guild damage abilities, but I still dont see her damage being on par with the other companions. I would reccomend maxing her heal (not intervention) and her barrier armor passive, and then investing in damage. I wouldn't bother getting might or sacred flames until much later as it will eat up mana that she could otherwise spend on healing. Invest in endurance, agility. and personality as priorities IMO, get as much life, armor, and mana as you are able.