Data Informed Decision Making (DIDM) has received considerable attention in education systems internationally due to increasing demands for school accountability and as a strategy to improve student outcomes(Schildkamp et al., 2013a). The pervasive nature of data available in schools appears to obscure fundamental consideration of what data schools find most valuable and what data based practices teachers and leaders engage in (Lawn and Ozga, 2009). Research indicates school leader’s attention is often on overall organisational performance while teachers focus more on student performance in class (Schildkamp et al., 2013b). Data use is enabled or constrained by certain school organisational and contextual conditions such as the nature of the data, leadership and school organisation (Schildkamp et al., 2013a). There has been very little research on this phenomena in Irish schools and this research investigated the type of data used and how they are used in seven schools identified as proficient in the use of data. There was a propensity to view the concept of data use, primarily in terms of assessment data to the exclusion of other forms, however, while state examination data was dominating impetus, routine classroom assessment received comparatively little attention. Similar to the international literature, principal leadership was the most influential factor contributing to data use in these Case Study schools. This involved building a shared vision, fostering a collaborative culture, distributing leadership responsibilities and creating an environment conducive to data use. While most of the principals were highly technically competent, there were deficiencies in the assessment and data capacities of the staff in general. There is considerable scope, therefore, to integrate data use further into the organisational culture of the schools and especially, increase the formative use of student learning data in teachers practice.