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.
You can find the current AdiIRCAPI.dll in your AdiIRC config folder or here, this is the glue between AdiIRC and the plugin.
Plugins can load and use any DLL's including referencing all .NET frameworks DLL's and do P/Invoke, just like a regular .NET program.
If you get a "System.NotSupportedEception: An attempt was made to load an essembly from a network location" error, try unblock the dll file as shown here here
Added in 1.8
/plugins
Gives focus to the Plugin Manager, opens a new Plugin Manager if it's closed.
See also Plugins Dialog.