KildareStreet.com is a modified installation of the TheyWorkForYou.com source code, which is maintained by the lovely people from MySociety, a UK-based charity. If you're looking for a place to start building your own copy of this site for your home country, we strongly urge you to go there and start with the 'canonical' version of the application.
It is available under a BSD-style license, which you can find here. Roughly, this means you are free to copy, use, modify and redistribute the code or binaries made from the code. Commercial or non-commercial use is allowed. However, both its original authors and ourselves disclaim any warranty, and you are expected not to use the names of MySociety or TheyWorkForYou without permission. You may not use KildareStreet's name without permission either.
We feel it's only right that we share our modifications. You can find a tarball of this site's code as it stood on April 16th, 2009 by clicking on this link here.
How on earth do I use this code?
Probably best that you ask on the mySociety developers-public email list. But again, don't use our modified version. Start with the original.