Irreversibilities in Stirling cycle coolers are discussed with reference to designing machines for operation at warmer temperatures rather than cryogenic applications. It is shown that the sensitivity of the cycle to some losses is dependent on the temperature ratio. The free-piston configuration employs a linear motor which greatly reduces side loads and facilitates the implementation of non-contact gas bearings. Mechanism efficiencies are therefore much higher than in crank machines which leads to an overall performance advantage. It is expected that an optimized free-piston Stirling refrigerator cooler could exceed 60% of Carnot overall (COP ≧ 2.3).