If the main symptom you are looking at is anger, then I would personally be hesitant with the bipolar diagnosis. I'm not sure if you've looked at the symptoms for mania, or hypo-mania. Irritability and anger can certainly be symptoms of mania. What generally goes along with mania, or can go along with mania is hyper sexuality (a way increased desire to have sex), increased energy, flight of thoughts leading to a difficulty concentrating, euphoria or just feeling real good for no particular reason (being high on life to an nth degree), grandiose thinking (thinking you are basically the best thing since since sliced bread and a real high opinion of yourself), and it can go on and on. If you have experienced any of those things in conjunction with the irritability then you might have some bipolar going on. I'm less familiar personally with hypo-mania than I am with mania, because I have bipolar type I. But as I understand it, hypomania is a milder form of mania. But also, it is important to note that if your periods of anger are being caused by, say, the alcohol like you mentioned, that isn't valid for bipolar. It can't be caused by a substance. There really needs to be a combination of things going on, and for a few days or more, for it to be something diagnosable.

Also, I don't think age 40 is a typical age of onset for bipolar. I did a quick search just now and I think mid to late twenties is what may be considered typical, but taht doesn't mean you can't have a late onset. So I hope that stuff helps out. Feel free to post any more info and questions.