BACKGROUND: We investigated the knowledge of ionising radiation among medical students and junior doctors in Ireland and assessed whether this knowledge improved with clinical experience. METHODS: A total of 269 subjects completed a questionnaire on the fundamentals of diagnostic imaging and patient doses. RESULTS: Overall knowledge was poor, 99% of subjects underestimated the dose of radiation involved in a barium enema, plain film of abdomen, lumbar spine X-ray and a PET scan. Almost 90% underestimated the dose of a CT abdomen/pelvis. 42% of subjects knew that PET involved ionising radiation while 27% thought that MRI did. There was a significant improvement in understanding after transition to a clinical environment, however, no further development. 1% had attended formal radiation protection courses. CONCLUSION: The knowledge of basic radiological procedures and patient doses was extremely limited. Current undergraduate teaching needs to be expanded and continued post-qualification to improve core understanding and facilitate safe practice.