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If you are eligible for the British Irish Visa Scheme, your visa will have "BIVS" printed on it.
With this visa, you can visit the UK from Ireland and remain for the remainder of the 90 days stay you were granted on arrival into Ireland, or you can return to Ireland as well. In either case your total stay is limited to 90 days from when you first entered Ireland.
Also be aware that under this scheme, you must always enter via Ireland if you have an Irish visa, and then travel to the UK. You will be refused entry if you attempt to travel to the UK without first entering Ireland. This also means you should avoid booking tickets to Ireland which transit a UK airport. (The reverse is true if you hold a UK BIVS visa.)
See the British Irish Visa Scheme Information Note published by Ireland Department of Foreign Affairs for complete information about the program.