Whether you need a visa to visit Britain & Ireland depends on your nationality and the length of your stay.
Citizens from many countries, including the USA, Canada, Australia, New Zealand, and most EU nations, can visit Britain and Ireland for tourism without a visa for up to 6 months. However, you’ll need a valid passport that’s effective for the entire duration of your stay.
If you're from a country that requires a visa, you will need to apply for a Standard Visitor Visa for both Britain and Ireland before your trip.
For the most up-to-date information, check the UK and Irish governments’ official visa and immigration websites, or consult with your local British or Irish embassy or consulate before booking your tour.