As far as we know, Irish chariots were used in the following manner:
Move onto the field with driver and warrior.
Warrior throws some spears and hops off the chariot.
Chariot moves off the field.
Warrior kills people in teh phace.
Warrior gets in trouble.
Chariot picks him up.
Warrior moves off the field to rest, reload, whatever.
Rinse, repeat.
Here is a great article concerning chariot warfare written by one of our historians at WFGs:
Chariots in Warfare and Entertainment
Feel free to use any of the articles as a reference for your mod.