Irish and Scottish folk music is very popular here in Germany, but the German music scene is mainly in concert format, folk clubs, culture centres, or other organised events, a few of the Irish pubs have sessions, but spontaneous music in pubs is rare. I don't know anything about the scene in Cologne, but I have played several times in Munich, but always at organised events. The Irish Folk Pub in Munich has live music every Thursday and I think is welcoming to visiting musicians on other nights. If you are in Munich is September the place will be packed for the Oktoberfest. Alanabit lives in Cologne, perhaps he might know of something there.
All the best,