lol This is my quick theory:
After his son dies and his withdrawal from Ba Sing Se, Iroh returns to the Fire Nation to find his Brother sitting on the throne. I think Iroh figures out what happened, but does not challenge Ozai. Instead, he hangs around the Fire Nation as the 'old tea loving general' until Zuko is banished. besieged by memories of his own sun, Lu Ten, Iroh accompanies Zuko in hopes of teaching Zuko his history and his true destiny. Now, however, IROH IS OUT THERE, ARMED AND DANGEROUS!!! lol idk what he's up to now.