How do I descend the skull cave quickly?

It seems like you're looking for the fastest way to get through the Skull Cave, so you better start saving up because this is going to be expensive.

I assume your Mining level is at least 2 and you have unlocked the crafting recipe for the Staircase. A Staircase is crafted from 99 stones, and will, when placed in the mines or the Skull Cave, turn into a set of stairs just like the ones found under rocks or spawned when enemies die. Stones can be bought from Robin at 20 gold each, resulting in a price of 1980 gold per Staircase. You can, with enough money or patience, completely fill your inventory with stacks of 999 Stones, enter the Skull Cave, and immediately craft and set down a new Staircase every time you go down a level.

With this method, you can reach level 400+ of the skull cave without mining a single rock or defeating a single enemy. That should get you plenty of Iridium Ore.

Another fast method for descending, that is also useful for when you reach the floors with Iridium, is to use Mega Bombs to clear rock.

Mega Bombs can either be crafted with 4 Gold Ore, 1 Solar Essence and 1 Dark Essence, or purchased for 1000 gold each from the Dwarf in the Mines. Purchasing them for 1000 Gold is far cheaper.

When blowing up rock, bombs function exactly the same as a pickaxe. They have the same chances to spawn Ores, Geode, Stone, and Stair Cases. Simply place a Mega Bomb in a group of rocks and you should get a stair case. Then, when you get to the floors with tonnes of Iridium ore, simply blow them up with a Mega Bomb to collect them quickly.

The other advantage to this method is that you can occasionally find a hole rather than a stair case. Holes allow you to travel several levels at the same time. The two I have used went 11 floors and 6 floors respectively.