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.
Plugins can load and use any DLL's including referencing all .net DLL's and do P/Invoke, just like a regular .NET program.
Added in 1.8
Gives focus to the Plugin Manager, opens a new Plugin Manager if it's closed.
See also Plugins Dialog.