The use of specialised prison units (SPUs) as a means of countering violent extremism and radicalisation is growing in popularity in many jurisdictions. Yet, little is known about their ability to prevent radicalisation in the long term, limit the spread of violent extremism, or promote de-radicalisation and disengagement. This paper begins to explore these issues by reflecting on the over 40 years of experience that Northern Ireland (NI) has had in this area. It is argued that the international community tends to have an incomplete understanding of: a) why SPUs were created and installed in NI; b) how their regime, conditions, and underlying rationale changed over time; and c) the long-term consequences that SPUs have had on the spread of violent extremism, de-radicalisation, and disengagement. This paper seeks to make an original contribution to this literature by highlighting the role psychological, situational, social, and political factors played in shaping the effectiveness of SPUs in NI. By providing a more in-depth analysis of why SPUs in NI entrenched extremism rather than promoted disengagement or de-radicalisation, other jurisdictions will be encouraged to reflect on how these factors may affect the success of their own SPUs in countering violent extremism.