Funny money?

I`ve worked for many, many years with money, and still keep finding odd things crop up.
Today I saw something I`d never seen before – a £100 note.
Yes, I have put the right number of `0`s on that. I didn`t know such a thing existed, I looked twice at it. Was it a joke or toy money? – No it felt like a real note and seemed to have all the right characteristics. So, yes, one of the cashiers had taken a £100 note. On closer inspection I discovered that it is an Irish note, Belfast to be exact.
Now, whether its legal currency in England, I don`t know, and no-one else is sure either – so we wait to find out.
(Woe betide the cashier if it`s not!!)