Background <p>Since the abolition of EU milk quotas in 2015, the Irish dairy industry has expanded with a 16.6% increase in calf births since 2011. Calf losses have major implications for the economic viability and sustainability of dairy enterprises. There is a paucity of literature on mortality in calves from birth to six months, particularly at a national herd level. Previous studies have tended to focus on herd size as a risk factor for calf mortality rather than the possible influence of herd expansion. The purpose of this study is to quantify any association between dairy herd expansion and the risk of being classified as a high or low calf mortality herd based on analysis of national dairy herd identification and movement records in Ireland from 2016–2020.</p> Results <p>In calves aged under 6&#xa0;months, herds that expanded &gt; 20% (OR 1.23 95% CI: 1.10–1.37, <i>p</i> &lt; 0.001) and &gt; 45% (OR 1.22, 95% CI:1.09–1.36, <i>p</i> = 0.001), were more likely to have &gt; 10% herd calf mortality, compared to herds that did not increase in size. Newly established herds were more likely to have poor mortality outcomes (OR 2.44, 95% CI: 1.82–3.29, <i>p</i> &lt; 0.001) compared to herds that did not increase in size. Herd expansion &lt; 20% was not associated with mortality outcome. Herd ordinal location and herd size were significant risk factors for &gt; 10% herd calf mortality.</p> Conclusion <p>This study has demonstrated that herds that have expanded &gt; 20% over five years and newly established herds were more likely to have poor mortality outcomes. It suggests that new entrants into dairy farming may benefit from targeted emphasis on herd health management. The results also highlight the value of national data as a tool to determine optimisation of farm interventions and surveillance and policy decisions to prioritise animal health and welfare.</p>