First of all, quality beats quantity when it comes to units. The AI will always produce more armies than the typical player, but they will also be of a lower quality: AIs don't always train their units in Fortresses and don't focus on training the better custom units when they are available.

Yithril is one of the most aggressive default factions though, so it's pretty normal that it has a decent army early in the game. Some tips on fighting it:

- Make custom units: defenders with a high defense and high health as meatshields + ranged staff users with extra elemental damage and extra initiative. Yithril has no ranged attacks (apart from spells or ranged attacks from heroes), so you can just retreat to the back of the tactical map and spam your ranged attack while your melee units are on defense (you can even use Guarded Strike if the enemy finally closes in). Preferably with a good commander hero.

- You can debuff the enemy armies before engaging them. Freeze or Tremor can also hold them in place for a while and give you some extra time to prepare. Tornado is even better, but you usually don't have that in the early game.

- Always focus on the highest threat. (Jugg)

Unfortunately I seem to be getting overwhelmed in terms of both quality and quality. I am not spamming pioneers. Should I be? 

I tend to be a builder and defensive type player in 4x games which is why the Ironeers appealed to me. Are there any race-specific tips/oddities I might be missing?

I'm really enjoying the game but I can't remember having so much trouble with a particular faction in other similar games in the past. 

Both quality and quality? Man, your troops must be really terrible...ok, lame, sorry

Unfortunately, as Gilden vs Yithril you are basically battling your evil twin. Both factions are designed to build large armies and conquer the world with raw might. No magic, no diplomacy, no gallivanting with heroes. You can field heavily armored, high HP troops, while Yithril can support many troops and rush you. This is the most straightforward matchup there is. If you are losing it, you might want to try some easier matchups first.

Just get some more experience with the game.

Maybe try a different race, it might suit you better.

I wouldn't list Gilden as one of the best default races, personally, so handicapping yourself could have to do with it.

I would say to make a custom race.  Pick traits that sound like something you think goes along with how you want to play.

I would say to avoid Ironeer blood if you do, tactical spells are really good, especially against Yithril.  Slow and Shrink both reduce their damage a lot.  Hasting yourself works wonders too.

You kinda shoot yourself in the foot trying to play Ironeer as a magic heavy race.  They are totally not built for it.

You would do better just picking Capitar race and the rest of the stuff the same as what Ironeer gets standard.

Guard Strike is really good, but not good enough to make up for Ironeer blood.

You might want to think about using Quendar blood also.  Quendar lets you train slave units which are quite a bit worse than regular units, but they have 0 upkeep so you can put 9 of them in each city.  That makes your cities much harder to conquer.

If you don't know what snaking is, look it up.

I wanna make sure, are you aware of blacksmithing tech? It basically unlocks everything you will need to survive until late game. Boar spears, iron golems, etc.  If yithril sends their heavily armored oppressors something, has 80ish def everytime it standby, you can safely ignore it until you're about to engage it.

And I think Yithril tend to rush through the military tree until they have their third weapon like pikes and plate armor.

I've won plenty of battles where they thrown plate heavy army against me when I still only have boar spears + leather armor and won. Very doable because of Life magic's heals.

Clever tactics.

Expansion is everything.

You can never build too many buildings. You don't need a lot of units, just better ones. Personally, I never build any units except in emergencies (random events usually) to hold off a monster.

Elemental damage ignores defense, so if Yithril's troops wear a lot of armor, your Mages will still do the same damage. Besides, they are ranged, so you can pull tricks like Raiddinn describes. Shielded tanky guys are very good, as you can build them for cheap and they are even tankier than armored units from other factions. However, to use their shields to full extent, you should not attack with them, except for Guarded Strike. Otherwise, they will lose their Defense bonus. So you will need some other troops that deal damage.

If you like a Mage and the Magic techs, I would strongly suggest another race. Mages are more powerful than the other classes, except for Ironeer Mages Try playing as Pariden or Altar or create a custom faction and take some strong traits: Altar or Mancer race; Enchanters and Master Scouts.

I have a suggestion. Considering the splash damage caused by juggernaut attacks, try to draw their main melee units which most of times contain a juggernaut, towards an expendable lone unit. Juggernaut will harm his own troops almost as much as it does to your unit but positioning that unit is crucial for this strategy.