It actually is the good old and venerable IRCNet. The oldest IRC network still active (as far as I know). Most better IRC clients already come with a preconfigured list of possible servers. Pick one near you. It doesn't need to be one of the mentioned German ones.
Simon Sunnyboy/Paradize - http://paradize.atari.org/Stay cool, stay Atari!
1x2600jr, 1x1040STFm, 1x1040STE 4MB+TOS2.06+SatanDisk, 1xF030 14MB+FPU+NetUS-Bee