In Ireland and the UK our winters can be very changeable. One day it will be sunny but cold and the next day it can be mild and very wet. There will also be plenty of wind thrown in for good measure. To allow for such variable conditions without needing to take over the spare room with cycling gear there are a couple of basic items of clothing that you will need.
A good base layer
I normally wear a short sleeve base layer underneath a warm long sleeve jersey for anything between 7 - 10 degrees which is often the temperature in wintertime in Ireland, below 7 degrees I wear a long sleeve base layer underneath a thermal winter jacket.
I used to often wear leg warmers under my winter longs, but technology has improved and the better longs are now plenty warm for even the coldest of days in Ireland.
A good all-round winter jacket
When you are out in changeable conditions with temperatures that can vary by up to 10 degrees during a 3 hour bike ride, having the right winter jacket can make the world of difference. You need something that is fully windproof and also something that will keep you dry if you get caught in a few showers of rain. It must also be warm enough that a zero on the thermometer doesn't look like an alarm bell.
A warm pair of bib-longs
During the winter if your knees are not well protected by something that is actually windproof it can lead to injury. With the windchill factor on a bike your legs can get cold easily and this is not a great feeling. I really like the new fabrics available. Mostly during the winter I will wear a pair of full membrane bib longs that are water resistant, windproof and thermal. I never get cold in these. The good chamois is another bonus.
These were not around much during my racing days, but now I seem to reach for a gillet 50% of the time I go out on the bike. I go over and back between 2 styles. On some milder winter mornings I like the full membrane thermal option. With the rear pockets it is as warm as a jacket on my body and I can wear a warm long sleeve jersey underneath, so that when the boys start racing on the way home to Carrick I am ready for action.
I also often use a wind and waterproof gillet that is really lightweight. This is great to wear for the first 15 minutes or so whilst you are getting warmed up heading down Seskin hill, and then can be taken off and folded up to be stored in a pocket. It can also come in handy during a shower of rain.