I think the awkwardness around telling him about your depression is less of a problem than taking it out on him when you are angry. I am not sure if that comes out right or not -- just that he probably would be more upset about the way he is treated than the fact that you are depressed.

Apologizing for being irritable will be well-received. Maybe you can brainstorm some things you can do when you start to feel irritable. I hope your conversation goes well if you decide to have it -- since he has known you for so long, it may not be a surprise to him.