Increasing electricity interconnection is necessary for both closer market integration and facilitating more renewable generation across the European Union. However, progress on the ground has been slow. This paper adds to the literature on the politics of interconnection through a case study of a proposed link between Ireland and Great Britain. We analyse how the institutional setting for investment has evolved at EU and Member State level, and how the influence of stakeholders is mediated by the ability of actors to define their interests and by institutional points of entry. Using this framework we examine the Greenwire/Greenlink project. We find first that developments in the institutional context, especially the adoption of a new cap-and-floor regime for merchant investment by the GB regulator, have had both positive and negative impacts on the project. Second, we find that regulators and governments in the two markets, have found it hard to define their interests in the project, and its net social benefit, due to informational complexities and changes in information over time. Third, the project has met with local public opposition that is strong but which has little leverage due to a lack of institutional entry points.