Barman and a pen

[rebbonk]

A barman finds a very expensive pen lying on the counter and the only customers near it are two drunk Irishmen.

He walks over and says "Does this pen belong to any of you gents?"

One of them looks and says "Giz it here a minute." He proceeds to write something on a beermat with it then says, "Aye, that's my pen."

Barman says "How do you know it's yours?"

Irish lad points to the beermat and says

Spoiler:

"'Coz that's my handwriting."