Be careful with the use of GB&I terminology here.
In GB&I the I’ usually refers to the RoI, ie the Republic, whereas ‘GB’ refers to England, Wales, Scotland and North Ireland although sometimes the latter is referred to as GB&NI!
As to greenfees, my recollection is that in three of the elements of GB (ie E,W and S) all visitors pay the same greenfee (see below though).
However, on the island of Ireland members of any club in the Republic or NI received a discount on a visitor greenfee, which effectively means those from overseas, ie those not a member of club on the island of Ireland, pay a premium.
There is also a slight complication in E and W where if you are a club member in E and W (not sure about in S) you can receive a discount through a scheme known as the County Card. This often, but not always, allows members of E clubs playing in E a discount, although a member of a club in E doesn’t get a discount in W (and vice versa).
Clear as mud!
Atb