Hirman (hirthman, or hirdman, as the more modern spelling goes) were originally personal guards, but over time became known as royal guards, very much comparable to the Huskarl (as both of their names imply "Household" in it). The absolute meaning of their name changes from different sources, however, it is mentioned quite often in Norse  sagas as 'great retainers', which could suggest they could even be vassals or even so much as 
<link="proto:Thegn">Thengs</link>.