Ranman,
Depression tends to be seen largely as a mental illness which causes overwhelming feelings of sadness. This is not always so; in neurological terms, depression can be considered an abnormal set of emotional responses to outside events. Which means of course that with some people it makes them very irritable and snappy, rather than sad as a primary symptom. Me, I get weepy, my brother, he glowers like a thundercloud and rumbles like a volcano. Same parents, same upbringing, same faulty chemical - different feelings. Only the despair is the same. Go figure... 
There are various cognitive tricks you can try to "throttle back" on your temper. One is to find something that provokes tender feelings in you - the classic one would be a photo of an infant son - and use it to calm yourself down when neccessary.
IMO, anything that makes you think "Things are not all bad", helps to "zoom out" from the worm's eye emotions that are depression, and gives you an emotional distance, an intellectual perspective, to help you control yourself. 
Another trick I know, - and I don't know how well this one would work with the "Mr Angry" mode of depression - is to start making lists of the things you do like about life. The idea here - and I know it sounds very Pollyanna-ish, and I wouldn't blame you for thinking so, but by God it works - is to start you noticing things to be happy about, rather than things that get you all wound up. Even supposing you are just jotting down "the smell of a new car makes me feel rich", or "sex is good" or "children's laughter makes the world better", being able to recall the emotions evoked by these sensations is an emotional counterbalance to the out-of-kilter depressive feelings. (One thing that is universal in people with long-term depression is that they all have damage in a part of the brain that weighs up events emotionally, and this affects the way they think about things, particularly memories.) And when depressed there is a tendency to underrate your own assessments; but if you write these things down when you are experiencing a good emotion, it is very hard for you to deny your own experience. You are almost forced to believe in your own positive feelings.
Hopefully, the angry moments you get can be defused ahead of time by using this technique as a counterbalance to developing moods, or at least to help break the anger - guilt - anxiety - anger cycle that can develop.
Half the problem with what I call "creeping depression" is when the feelings sneak up on you so quietly that not only your mood changes, but also your style of thinking and therefore your attitudes. In other words, the tail starts to wag the dog rather than the other way around. The trick there is to get your head round the fact that you are not feeling this way due to events, you are feeling as you do because your brain chemistry is screwing up. Confused ? Hurt ? Angry ? In despair ? Life doesn't make sense ? That's because depression doesn't make sense, any more than lumbago is your back making sense. If you have UC, it may be that you are depressed simply because your gut is malfunctioning (it shares neurotransmitters with the brain).
Some anti-depressants - notably the SSRIs like Prozac and Seroxat - can provoke mood swings, particularly towards anger; if you're on one of them and that's what it's doing to you, my sincere suggestion is, do your absolute utmost to get yourself on some other medication as a matter of urgency. Things won't get better, and they may get hellishly worse. 
I'm sufficiently certain of this that I'm not worrying about legal action here. Much...
Having said that, one of those drugs made me very angry, the other cured my problems and fixed my life. It's very much luck, and I for one am glad the drugs companies buried the adverse drugs trials data in order to get them to market, for all that I'm sorry for the unlucky patients.
And I certainly would never stop taking any drug without taking medical advice, as stopping suddenly can have drastic effects.
I know that I have read posts on Healingwell forums about the different drugs and also about cognitive therapy, but unfortunately I cannot remember where, I lurk so much - have you tried the Search facility ? For that matter, have you ever tried a full spectrum light, in case you have Seasonal Affective Disorder ?
Hope this helps Ranman. But if worst comes to worst, remember - it's not just good times, it's also the hard times, when we have to learn to cope with harsh reality, that binds families together.
Merry Xmas, and all the best for 2005.