Hey - as Zaf and Cornish have both pointed out, all AD's take a while (4-6 weeks) to really take effect. Depending how long you have been on them you could be experiencing some early side effects and if this is the case then they
should pass over the next few weeks.
I was on Mitrazapine for a while last year - Like Cornish says, the only thing I really noticed was that my appetite increased dramatically!! This was the main reason I came off of it - I get very anxious about my weight, so the fear of putting weight on just made my situation worse.
As I am sure you are well aware - extreme tiredness is a symptom of depression - confusingly, a lot of AD's have a side effect of 'tiredness'

So... only you will know if this and the irritability are things that have coincided with taking the medication of if they are symptoms of your depression. If they get worse or to the point where you are finding them unbearable I would encourage you to go back to your gp.
Take care and let us know how you are getting on.
Nay x