This was my question, and I have to confess that for me it was an either/or: is it Belfast, or is it Derry/Londonderry?
Fortunately (for me), I went the right way.
HM Prison Crumlin Road closed its doors as a prison in 1996, and was empty for many years. A restoration project was announced in August 2010, and in November 2012 the prison opened as a tourist attraction and conference centre. It now also hosts concerts.
As far as I can make out, the nearest prison to Derry/Londonderry is Magilligan. This is a medium-security prison, on the peninsula that forms the south-eastern (true right bank) 'jaw' of the seaward entrance to Lough Foyle. (Lough Foyle is the estuary of the River Foyle, which flows through Derry/Londonderry before forming the Lough.)
HMP Magilligan is about 20 miles from the city of Derry/Londonderry.
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