I am looking for a way to resolve a relative url the way you would with a page or control instance (MSDN Docs) such as:
Page.ResolveUrl(\"~/common/Error.aspx
Use something like this - Controls is a folder name in your application and myController is the controller name. to create and instance and load the controller you can do it by:
Controls_myController ctrl = Page.LoadControl(Page.ResolveUrl("controls/myController.ascx"));
Hope this helps.