But I have to wonder -- if he's commanding a fleet of ships, why is he General Iroh instead of Admiral Iroh?
There are a few possibilities for why they would have chosen a ground forces office:
A) Iroh is a member of the Fire nation's royal family, and the forces may have been made up largely of Fire nation soldiers/ships.
B) They may have mistakenly believed most of the fighting would actually take place on land and gave Iroh command early out of a desire to maintain steady leadership throughout the battle.
C) Iroh may be acting in a manner similar to the Chairman of the Joint Chiefs or perhaps a Commander-in-Chief and needs to be on-site due to technological limitations in their long-distance communications methods.
D) The original intent may have been that he act as a figurehead. An admiral could be giving most of the orders while they're at sea.