The aim of this article is to examine how a group of Polish male prisoners negotiate daily life in prison custody in Northern Ireland. In a jurisdiction emerging from years of armed conflict, the prison system is currently undergoing structural changes to bring it in line with other systems providing peacetime custody. Alongside that reform, another transition is evident, that of a rapidly increasing diversity of the prison population. Unlike elsewhere in the UK, the number of foreign national prisoners in Northern Ireland began increasing only in the last decade. Analysing the ways in which Polish prisoners negotiate relationships with other prisoners and staff, the article concludes that many live in a prison within prison, with a high wall of communication barriers around them, suspended before their entry into custody and the ever-looming moment of deportation. The prison system, largely unprepared to deal with more diverse populations, facilitates their existence in ‘mono-cultural boxes’ in the meantime.