All the plugins available for IrfanView come with their own installer. By default, the packages are developed to copy the necessary files into IrfanView "PlugIns" directory. If you have such plug-in, launch its installation package and it will copy all the files where it's required. Restart IrfanView and the plug-in should be available for using.
There is also a page created by the developers where you can acquire various plug-ins.
Plugins page: http://www.irfanview.com/plugins.htm