Cache is on the way up to "Parco Archeologico fenicio-punico di Monte Sirai", the best place to visit in the area. Park here to admire the view and behind you is the cache! You don't have to enter the site to find the cache (although you should)
http://www.sardegnaturismo.it/en/point-of-interest/monte-sirai-area
http://www.sardegnaturismo.it/it/punto-di-interesse/area-di-monte-sirai