heat pumps

These work like a fridge in reverse and can be installed underground, in water or to air.  Renewable Heat Incentive payments are available for accreditted installations.
They can work well if the building is properly insulated and with either underfloor heating or large radiators but they do require electricity to run them so need to be setup and run well..
Ideally they should be powered by a renewable source and used in conjunction with a wood stove.  Air source heat pumps suffer from a drop in efficiency in coldest weather, ground source are more consistent providing the ground loop is large enough.
Air source 'air conditioning units' are fairly common but not often used domestically and if used in summer they can end up consuming more energy than normal electric heating, they are not eligible for RHI payments..

 see advice on when heat pumps are suitable here http://davegreenenergyservices.yolasite.com/heat-pumps.php.