I've had the exact opposite effect from Melatonin, I've been on it now for the past 3 weeks or so. Hell, people at work have even commented on how much more relaxed I am recently. Normally I am pretty short fused.
I do find if I go above 3mg I have trouble sleeping.
Have you tried eliminating just the melatonin to see if your depression subsides?
Anything else going on in your life that could be causing/adding to it depression/sleeplessness/irritability?
Also, are you sure you are actually 'sleeping' on melatonin? Are you waking up refreshed? or just as tired as when you went to bed?
EDIT: Just found this link:
Melatonin & Depression