We saw before that the dental/cononals/cadad rule debars the historic juxtaposition of dental fricative and dental stop in certain examples, like *ag an thine (ag an tine) for lenition, but what stopped eclipsis in *ag an ndoras?

cadad disallowed séimhiú-ed dental after a dental. However in case of urú you get another dental sound, so cadad doesn't work. Look - there is nothing wrong with n+n combination "ag an ndoras", or n+d "ag an dtine".