Copper 27 mm (size of halfpenny) 4.03 g (weight of farthing) George III observe Hibernia harp 1760 reverse But I find no record of George III on a 1760 coin even though he became king late that year. What is this? Is it common?
My understanding is that the first regal George III Irish halfpennies weren't struck until 1766. That data point, coupled with your specimen's light weight, suggests that it's a contemporary counterfeit.