See here for native DLL support.
See http://www.adiirc.com/docs/ for API
Docs can also be downloaded as a Help file here http://www.adiirc.com/docs/Documentation.chm
If you want an API for something not covered by the current API, please open an issue.
Added in 1.8
/plugins
Gives focus to the Plugin Manager, opens a new Plugin Manager if it's closed.
See also Plugins Dialog.