With much of the literature on school self-evaluation (SSE) stressing the importance of data use, this paper explores how teachers in Irish post-primary schools are coming to terms with this new challenge. Since 2012, all schools in Ireland are required to engage in SSE for the purpose of improving student outcomes. For the first time, teachers and school leaders are being asked to systematically gather and analyse various types of data, devise improvement plans and implement improvements. It is interesting to explore the introduction of a school improvement process that requires data as evidence for self-evaluation, but where very little data currently exists compared to other jurisdictions and where the discourse of data use in schools is relatively new. This research outlines the experience of 13 post-primary schools that were supported by the DCU Centre for Evaluation, Quality and Inspection to complete an SSE process, during which, each school gathered and analysed a range of data. This study is part of a larger action research project which explores various aspects of the implementation of SSE in schools, including models of support and continuing professional development for schools. This paper looks specifically at the use of data by the schools involved. The key research questions ask: what data was gathered by the schools and what was the attitude to and experience of data-use among teachers? In doing so, the research explores some of the current research questions in relation to data use in schools. Overall, the findings indicate that schools gathered a range of data, which was mainly quantitative due to a focus on quantitative target setting. Despite a generally positive attitude to the usefulness of data and the skills learned, participants did not appear convinced that they would be involved in data use on an ongoing basis.