The Ironsouls are a race of redstone robots built by the Highborns, which are now Villagers. They are probably rare. They are found in the capitol. They are mentioned in AO1 by tucker, who wants to "find one and fix him right up" They are said to have feelings, and are more intelligent, almost like they have a soul. Which is probably where the term 'Ironsoul' was coined.
Not much is known about Ironsouls, though we know they were made by the Highborn.