RTÉ has a quite nice applet for the Windows Vista or Windows 7 sidebar. Unfortunately, the do not yet offer one for KDE and Plasma. Instead of waiting for the applet, I wrote a little web-radio applet by myself yesterday.
The plamoid has a simple list of radio stations and a play and a stop button. Easy, isn't it? Of course you can configure the list of stations. If you download and install it, it comes preconfigured with all the RTÉ stations and my favourite Austrian station FM4.
You can download the plasmoid from here: http://krename.sourceforge.net/data/plasmaradio-0.1.tar.gz.
To install the plasmoid, follow the following steps after downloading it:
tar xvfz plasmaradio-0.1.tar.gz
cmake -DCMAKE_INSTALL_PREFIX=$KDEDIRS ..
I would be happy to receive some feedback on this little piece of code. By the way, I think it really shows what great stuff can be done in so little time using the KDE4 API.