Wind power's share of the generation mix is increasing on most of the world's power systems. As the penetration of variable, non-dispatchable generators increases, power systems are becoming more sensitive to extreme weather events on the supply side as well as on the demand side. Sensitivity of supply to weather is mainly due to wind variability while demand sensitivity is driven by space heating or cooling loads. The interplay of these two weather-driven effects is of particular concern if demand spikes driven by low temperatures coincide with periods of low winds. The Republic of Ireland now has over 1.2 GW connected wind capacity, compared to a total dispatchable capacity of 6.3 GW, and a target of over 30% of electricity generation from wind by 2020. A further 0.3 GW of wind capacity is present in the interconnected Northern Ireland system. In December 2009 and January 2010 Ireland experienced a prolonged spell of unusually cold conditions. During much of this time, wind generation output was low due to low wind speeds. Figures for power demand, wind generation and wholesale electricity market prices during this event are presented as part of a case study of the effects of weather extremes on power systems with high penetrations of variable renewable generation. Despite the unusual meteorological conditions, generation capacity was found to be adequate to meet the demand peaks.