This dissertation uses mediation theory to examine the implementation stage of the Northern Ireland peace process. This highlights the fact that mediation does not end when a peace agreement is signed. The implementation of agreements is also a difficult challenge and an examination of how mediation theory can explain the role of third parties at this significant stage will fill a gap in our understanding of post agreement mediation. It examines how the Independent Commission on Policing, the Independent International Commission on Decommissioning, the Independent Monitoring Commission, and the Consultative Group on the Past managed their respective issues. The analysis establishes what type of mediation each commission used and how the identity of the mediator, the issue intensity, terms of reference of the commission, and the presence and nature of the Good Friday Agreement affected this. It finds that mediator identity has a strong effect on the type of mediation used; a combination of members with high international status and local members facilitated more interventionist mediation. Similarly, despite an inclination to focus on certain aspects of a mandate, terms of reference that explicitly provided for deeper involvement resulted in more interventionist activities. Issue intensity was not found to have a significant effect and its impact was largely mitigated by other factors. Finally, the Good Friday Agreement had a complex effect on the mediation. Mediation type was affected both by the existence of a peace agreement - which was seen as legitimate given its approval in a referendum - and the nature of the agreement as international, Lijphartian, and coercive.