it is two more hp points per ironflesh point. This might not seem much, but if you dont use light armor, this helps alot. i can feel the difference between 0 and 5 ironflesh and i use medium-heavy armor.
if you use heavy armor, every hp point is worth more (because the enemy does less dmg, so they take less hp points per attack). so basically, when you have soome ironflesh and heavy armor, you become a tank. dont use it with light armor though.
i always put ironflesh as last