This 2,000-square-foot home in Feakle, Ireland, is virtually free of carbon emissions, relying on photovoltaic panels, geothermal energy and wind turbines for electricity. A wood stove heats the structure, and residents rely on a rainwater collection system for even greater self-sufficiency. And with a listing price of $350,000, the energy savings aren’t the only thing that’s efficient about this environmental paradise.