The Hirdman is only accessible in the Expanded Mod where it appears as the Norse heavy infantry good against infantry, it is an extremely effective unit in this role, even being a decent unit beyond its intended role.
The Hirdman was the original Norse heavy infantry before being replaced by the Huskarl. It functioned much like the other swordsman/mainline infantry as a good-all round infantry unit, similar to the Militia line from Age of Empires II.
The Hirdman was also intended to be upgraded into a hero version of itself called the Hersir before the units were divided and the Hirdman became a Huskarl.
A detailed look at the unit's history is available here.
Due to be deleted before the creation of history files, the following is based on the Wikipedia article "Hird" it how ever has been condensed to be more accurate to Age of Mythology's history txt files.
"Hirdmen were the nucleus of personal companion bodyguards, they were essentially Elite Huskarls, and being mentioned many times in Norse Sagas, and law codices by the reign of Hakron IV, they became more of a Men-At-Arms, more or less a Formal version of what they once were"
- The texture map for the Hirdman, is located in the Multiplayer Alpha textures.bar file, under "Infantry N Hirdman Map".