See here for native DLL support.
See https://dev.adiirc.com/boards/4/topics/146 for more about plugins.
Added in 1.8
/plugins [-luixo] [filename]
With no switches, opens or focuses the Plugin Manager dialog.
-l - Loads a installed plugin file.
-u - Unloads a loaded plugin file. (Plugin file is still locked until AdiIRC is closed)
-i - Installs a new plugin to the plugin folder.
-x - Uninstalls a installed plugin in the plugin folder.
-o - Used with -i, allows overwriting a installed plugin, if the plugin is not loaded.
[filename] - Filename/path of the plugin.
; Installs the plugin 'MyPlugin.dll' from the folder 'C:\User<\Nick\Desktop' to the Plugins folder. /plugins -i C:\User<\Nick\Desktop\MyPlugin.dll ; Loads 'MyPlugin.dll' /plugins -l MyPlugin.dll