That's a neat find It's a British King George IV Irish Hibernia halfpenny or penny. I think a halfpenny would be in the 27mm range and a penny around 33mm or so.
it is a Irish farthings 1737-1738, 1744 and 1760. you would have to measure it ti figure out what one
21.57 mm would be the 1760
21.6 mm would be the 1744
22 mm would be the 1737-1738
see this link http://www.coins.nd.edu/ColCoin/ColCoinText/Br-IrishCopper.2.html