Iron ingots can be obtained in Minecraft Skyblock via 2 methods. 1) Zombies will rarely drop iron ingots upon death 2) It is possible to find blazes in the nether. Note: they will only spawn in nether fortress territory. The nearest territory is at approximately
X:76 Y:62 Z:63
After obtaining 1 or more blaze rods you will need to craft a brewing stand. You will then need to sacrifice 3 pieces of sand and smelt them to craft 3 bottles then fill them with water. After this, add the following ingredients respectively: sugar, fermented spider eye, gunpowder. This will give you 3 splash potions of weakness (you will need 2 at the minimum). You will also need to make 2 or more golden apples. Then you must find 2 infected villager zombies and heal them by splashing them with the weakness potion then right clicking them with a golden apple each. Once you have 2 villagers you put them in a confined space and place a large surplus of doors. This will make them breed and make more villagers which will higher the chance of iron golems spawning. You then must place a water stream that leads to a drop that is at least 54 blocks tall. Once lots of iron has been collected it is recommended that you also craft hoppers that collect the iron and flowers. You can also have villagers outside the iron golem farm so you can trade with them to get various resources such as diamonds and redstone.
These methods are up to date as of Skyblock 2.1, Minecraft 1.7.5