This is the Goalkeeper weapon system, seen here on the bow of HMS Illustrious.
Goalkeeper is a Dutch close-in weapon system, which defends ships against incoming missiles and ballistic shells. The system consists of an autocannon and an advanced radar which tracks incoming fire, determines its trajectory, then aims the gun and fires in a matter of seconds. The system is fully automatic, needing no human input once activated. The name comes from the soccer/football position. The system is made by Thales Navy Netherlands, and over 50 have been produced. The system can also be deployed to protect airfields.