This thesis focuses on Margaret Thatcher's involvement in policy making in Northern Ireland in the period building up to the Anglo-Irish Agreement (AIA) of November 1985. The findings in this research are surprising: that Thatcher's involvement in Northern Ireland, though intermittent, was much greater than has previously been appreciated, and without this involvement there could have been no AIA. Although we already know a lot from the scholarly research on Thatcher and Northern Ireland, the literatures are somewhat disconnected resulting in a distinct gap in the literature of studies looking at Thatcher's role in Northern Ireland matters. The rolling programme of archival releases under the 30-year rule provides an opportunity to reassess, and revise, what we know already about Thatcher and Northern Ireland in light of the new empirical evidence. This thesis unveils new information which transforms our view of Thatcher and provides new answers to existing questions on Northern Ireland. It fills in the gaps from during this period by focusing on the roles of the Prime Minister and Secretaries of State for Northern Ireland in greater depth than ever before. It will make a significant contribution to our knowledge by bringing new insights into Thatcher's relationship with the Irish Taoiseach, and her determination to do something about Northern Ireland, as well as her relationship with the Secretaries of State for Northern Ireland and key advisors such as the Cabinet Secretary Robert Armstrong. The primary research question that this thesis answers is 'To what extent was Margaret Thatcher involved in policy making in Northern Ireland, specifically from 1979 until 1985, and what was the impact of this?' In order to answer this question, two sub-research questions will also be addressed along the way. The first sub-research question relates to Thatcher as Prime Minister and her style of leadership. For the first time we have key respondent interviews with the full archival record to deepen and enrichen our understanding of the role of the key actors. This will help us to understand what sort of a leader Thatcher was - autocratic and ideological or pragmatic. The second sub-research question relates to Thatcher as Prime Minister and ministerial relations. To understand Thatcher we have to look at her Secretaries of State for Northern Ireland. We can learn about her leadership in Northern Ireland through these relationships, allowing an emerging picture of Thatcher to build up over subsequent chapters in this thesis. In order to be able to answer these questions, this thesis combines rigorous archival research, using newly released government archives under the thirty year rule, and an extensive programme of elite interviews with insights drawn from a political science literature on the core executive. This allowed me to construct a robust historical narrative informed by inter-disciplinary insights, bringing the Thatcher and Northern Ireland literature together in a way which has not been done before.