I am a permanent resident in the USA. I have an Irish tourist visa. While in the Republic of Ireland, I would like to take a day tour to Giant's Causeway (Northern Ireland). Will I need a visa for the UK? I have applied for a multiple-entry visa for Ireland.
Your US residence does not enable you to enter the UK visa free. However, you would not need a separate UK visa if your Irish Tourist Visa is endorsed with the coding BIVS. The British-Irish Visa Scheme allows for mutual recognition of short-stay visas between the UK and Ireland. If it does not, you would have to apply for a UK Standard Visitor visa.