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Abstract Background Despite efforts to improve the accuracy and transparency of the design, conduct, and reporting of randomised controlled trials (RCTs), deficiencies remain. Such deficiencies contribute to significant, avoidable waste of health research investment and impede reproducibility. This study aimed to synthesise and critically analyse changes over time in the conduct and reporting of internationally published evidence on patient and/or population health-oriented RCTs conducted in one country. Methods This observational study drew on systematic review methods. We searched six databases for published RCTs (database inception to December 2018) where ≥ 80% of participants were recruited in the Republic of Ireland. RCTs of interventions targeted at patients, providers and/or policy makers intended to improve health, healthcare or health research were included. For each study, screening, data extraction and methodological quality appraisal were conducted by one member of the author team. Results From 17,560 titles and abstracts, 752 unique RCTs were published in 745 papers between 1968 and 2018, with a steady year-on-year increase since 1968. The number of participants was in the range of 2–8628. The majority were parallel design (86%) and classified as treatment evaluation. Of the 418 RCTs published since the introduction of mandatory clinical trial registration by the International Committee of Medical Journal Editors in 2005, 32% (n = 134) provided a trial registration number. This increased to 47% when taking studies published between 2013 and 2018 (n = 232). Since the 1996 publication of the CONSORT statement, 16% of included RCTs made specific reference to a standardised reporting guideline and this increased to 31% for more recent studies published between 2013 and 2018. Overall, 7% (n = 53) of studies referred to a published study protocol, increasing to 20% for studies published between 2013 and 2018. Conclusion Evidence from this single-country study of RCTs published in the international literature suggests that both the number overall, the number registered and the number referencing reporting guidelines have increased steadily over time. Despite widespread endorsement of reporting standards, reporting of RCTs remains suboptimal in domains such as compliance with the CONSORT statement and prospective trial registration. Researchers, funders and journal editors, nationally and internationally, should continue to focus on improving reporting and examining avoidable waste of health research investment.