This study describes a national data audit of the National Intellectual Disability Database (NIDD). The NIDD is a national information system for intellectual disability (ID) for Ireland. The purpose of this audit was to assess the overall accuracy of information contained on the NIDD, as well as collecting qualitative information to support the improvement in the quality of data contained on the NIDD. A nationally representative sample was generated from the NIDD. Twenty-five auditors were recruited from senior staff within statutory and voluntary intellectual disability services and trained by the authors. They carried out 250 clinical interviews with individuals with ID, their families and/or paid carers, using structured questionnaires. The data collected were analysed using frequency analyses and compared with the existing NIDD data set to assess accuracy. Qualitative information was also collected. Overall, the results from the audit indicate that almost three quarters (72.2%) of all the data recorded on the NIDD are accurate, with 19.3% inaccurate, and 8.5% of the sought audit data not returned. The audit found that the NIDD significantly overestimated the need for enhanced residential care services. The study highlights the need for clinician and service user involvement in specialist service data collection, in order to both conduct valid research and to best plan for ID service development. (Contains 1 figure and 1 table.)