The Stirling cycle is something that was formerly a novelty and academic subject that could produce almost no usable power. Modern technology has changed that. The Stirling engine in the Swedish sub Uppland, drives an electric generator to power the ship. This is similar to the way locomotive engines are powered (trains), the big difference is the performance. Submarines require similar amounts of power to function, but anyone who has heard a train knows that they are loud...
Every part of a submarine is designed for underwater stealth so it would make sense to have a power plant that was very quiet. Although most information is classified, one could bet that the 'Stirling' on this sub produces a lot less noise than its nuclear powered counterparts, that use steam turbines to turn a generator.

   A less classified and more economical use of Stirling engines is that of solar powered sterling engines. The focused light from the sun is the heat source and the surrounding air is the heat sync. These engines are designed by Kockums,  the same people that produced the engine for the Uppland.