Seemed like a good place to ask a question about plugin installation, as the name of the thread is "How to install roundcube plugins on iredmail?" I'm not trying to hiijack anyones topic. So my apology if the question came across like that for what ever reason.

To answer your question. In roundcube (web gui) the new version .7.2 , its recommended that all plugins are configured and installed via a tool called plugin_manager (a new plugin) . 

see
http://code.google.com/p/myroundcube/wiki/MyRoundcube

When installed it adds a gui entry to the settings page that manages which plugins are installed and active as well as lets you check if the proper version is installed etc.

The documentation for setup is not good. But its pretty easy once you figure it out.
1st upgrade to newest version of roundcube with instructions in this thread above.

If you wish to use the plugin manager this is what you have to do.
1. Download and unzip the bundle in a temp dir.
2. copy at miniumum  global_config and plugin_manager  plugin dirs to your /usr/share/apache2/roundcubemail/plugins dir. edit /roundcubemail/config/main.inc.php   adding the plugin_manager plugin
// should look something like this
$rcmail_config['plugins'] = array('managesieve','plugin_manager');

3. in each plugin folder do this  "cp config.inc.php.dist config.inc.php "
4. edit the configs put admin user and  file paths ect.: nano config.inc.php   
5. login to roundcube, now you will see plugin manager in list(section) in settings.
6. check update plugins and it will give you zips for current plugin builds, etc.

Works well after you get it. I installed plugins like rcguard for captia, some other stuff.
the calendar I can't get to work fully as that was what I was after (caldav).

that is the deal with plugins in roundcube 0.7.2 and plugin manager. if anyone figures out the calendar, well that would be great if you shared.

thanks!