Euro is the main currency you'll need. If your tour visits Northern Ireland, you'll also need Pound sterling (£).
There are two currencies on the island of Ireland.
Euro (€) is used in the Republic of Ireland (including most big cities: Dublin, Cork, Galway, Kilkenny, Kerry, Donegal, Mayo and Waterford). Flying into Dublin? You need Euro!
Sterling (£) is used in Northern Ireland (including Belfast, Derry, Newcastle, Bushmills and Giant’s Causeway).
You only need to have British pound sterling (£) if your tour is visiting Northern Ireland:
These tours go to Northern Ireland and require British pound sterling (£):