Skull Cavern, the deeper the better. You really don't start seeing them pop up until floor 30, by 50 they're likely encountered on just about every floor, and by 100 they're relatively common. There's many threads on the topic of how to delve more effectively in there, but I'll quickly try to sum up some of it:
Use Staircases: You can trade Jade for them at the Desert Trader, Crystalariums can get you lots of Jade, and if you save up a bunch you can delve quite deep quite easily.
Use Bombs: Way faster and Way more effective than pickaxes at breaking rock clusters, ideally bring 30-50 so you can just blow up big clusters on demand.
Use Speed Boosts: Spicy Eel is best, especially if you have Rubies to spare, along with Coffee or Espresso. Any speed boost is better than not though so if all you have is pepper poppers they'll work fine.
Be Lucky: Go on a lucky day per the fortune teller. The luckier the better. That's part of why Spicy Eel is good, plus there are some other ways to improve your luck.
On a casual delve (minimal staircase usage, just enough to escape purple mist or infested floors, with pepper poppers for food) I typically come home with 100+ iridium ore, on a serious delve where I use 50+ staircases off the bat, spicy eel, and use bombs more liberally, I typically come home with 250+ ore, so it's definitely an effective strategy. There are certainly others of course, but I think it's fair to call that strategy a common or popular one, and likely to be the most effective. Best of luck!