This study examined the prevalence and impact of moderate to severe RLS (MS-RLS) in primary care patients in Ireland. Patients completed a screening questionnaire and those with symptoms suggestive of MS-RLS underwent a diagnostic interview. Patients diagnosed with MS-RLS completed quality of life and sleep assessment questionnaires, and their medical records were examined. Of 2628 patients screened for RLS, 74 (2.8%, 95% confidence interval 2.2%-3.5%) were ultimately diagnosed at interview as having MS-RLS. These patients reported significant impact on sleep and quality of life; 24 (32.4%) had consulted a health care professional about their RLS symptoms but only 4 (16.7%) were diagnosed with RLS. Ten (13.5%) MS-RLS patients were taking inappropriate medicines to try to relieve their symptoms. Clinically significant RLS is common in Irish general practice and has a significant effect on sleep and quality of life. Nevertheless, the condition often goes undiagnosed.