In spite of being the Order's first (and, prior to the Fall, only) Shadeslayer, Irnstad never had the reputation of a great warrior.
Instead, him and Lenora his dragon were renowned for their ability to survive anything, from storms to cave-ins to seemingly mortal injuries.
Their fellow Riders would joke that if you locked Irnstad in a room with a Shade, after a few days, the Shade would beg to be let our because the damn human just would not die.
Such was Irnstad and Lenora's reputation that after their deaths at the hands of Galbatorix, the mad Rider had their hearts cut out and their heads removed, just to make absolutely certain they were dead.