Ballycopeland Windmill, the last working mill in County Down which, at the beginning of the 19th century, boasted over one hundred examples. Many of the mills in County Down, one of the major grain producing counties in Ireland, were sited on drumlins and some ruined examples can be seen at J5950 : Old windmill, Portaferry 
and J5866 : Ruined windmill near Greyabbey
This mill stands a mile or so west from the small coastal village of Millisle and was built c1790, remaining in use until 1915. In 1935 it was acquired by the government of Northern Ireland and is now one of the most important and interesting historic monuments in the care of the Northern Ireland Environment Agency Link
(Archive version: Link
) . Fully restored (the only working example in Ireland) it is usually open to the public, although when I have visited recently it has been closed for repairs.
A typical Irish stone tower mill, Ballycopeland has a moveable cap turned by means of an automatic fantail, ensuring that the sails are always facing into the wind. When the wind blows directly onto the sails the blades or vanes of the fantail are not moved; if the wind veers, it catches these blades and the whole cap turns slowly on an iron ring or ‘curb’, bringing the sails back into the wind. In addition to the ground floor the mill has three storeys – a ‘drive’ floor, ‘stone’ floor and ‘hopper’ floor, from the first to third level respectively. The rotating cap sits above the hopper floor. Three pairs millstones sit at the ‘stone’ floor – one set for making wheatmeal, one for shelling the grains of corn (a process which separated the useless ‘hulls’ from the valuable ‘seeds’ or grain) and a set for grinding grains into oatmeal.
The power of the wind is transmitted back from the sails or ‘sweeps’ by means of the central ‘windshaft’ passing back into the cap. On the inner end of the rotating windshaft is the great ‘brake wheel’ which drives a bevel gearwheel or ‘wallower’ perched on top of the main (vertical) driveshaft. This power turns the three sets of grinding stones below.
The sails would have been covered with canvas sailcloth. The amount of sailcloth used determined the amount of power and this would have been adjusted from within the cap, even while the sails were turning, to suit the particular grinding requirements at any given time. Ballycopeland produced oatmeal and wheatmeal for human consumption, as well as animal feed such as shelled and bruised oats, yellow meal (from maize), kibbled (crushed) grain for poultry and ground peas and beans.
Information above is mainly based on a text by W.A. (Alan) McCutcheon from the book ‘Some People and Places in Irish Science and Technology’ Link
See also Link
for some related images.