An Iron helmet is a vanilla item still contained within FTB. Iron helmets are used to give the player armor protection against falling and enemy mob damage.
An Iron helmet provides the player with 2 armor points. However, the Iron Helmet is subject to durability.
The Iron Helmet can also be enchanted, with enchants including: Respiration, Aqua affinity, Protection, Fire Protection, Blast Protection, Projectile Protection, Thorns and Unbreaking.
A list of enchantsments are found here.