As to your questions
1. Irritability can be a symptom of depression. Not always but sometimes. You may want to do some research online about the symptoms of depression.
2. Personally I still drink coffee. Not sure about the standard line on this, but I would not change my coffee drinking anyway.
3. The need for medication varies from person to person. If the depression is severe (suicidal, can not do anything, etc) the need for medication is higher.
4. Motivation for work is a personal project. No one has a sure method for motivation. What you need to ask yourself is whether the job itself is bothering you or else if your depression is hurting your motivation. If it is the job, I would look elsewhere. If it is depression, changing jobs will not change anything.
Good Luck