Sometimes called a tap, this is actually a coupler for Guinness kegs. This is the coupler only and it does not include a nut, nipple, washer, or beer dispenser.
Fits U system keg valves.
A non-complete list of brewerys and beers that use the U system:
Abby White, Guinness Stout, Harp, Kilkenny, Magners, Smithwicks Irish Ale