Ironite is the primary currency used in the store. It is awarded in small amounts throughout the game's levels, but missions, achievements, events and sacrifice can give you ironite in larger amounts.
You'll want to keep some ironite on hand for simple things like additional Sands of Time / Sands of War. Rare souls / event souls may also be purchased with ironite.
If you run completely out of ironite, it's possible to buy it with real world dollars, but most players prefer to keep those in their own wallets and not those of the game devs.