This tower is an example of the Napoleonic towers. These were built around the Irish coast in the beginning of the 19th century in response to the threat of a French invasion.
The signalling system enabled communications around the coastline between signal stations which were built to be visible to other stations on either side. Sending a message involved raising and lowering a large flag, a small pendant and four black balls. They were hoisted in different combinations on a system centred on a tall wooden mast. The stations also were able to communicate with ships.
After Napoleon's defeat at the battle of Waterloo the threat of an invasion receded and the towers were abandoned as their upkeep proved to be too expensive.
There is an informative article on Napoleonic Towers here