Background: Coronavirus disease 2019 (COVID-19) and the public health measures enacted to control its spread may affect the mental health of the general population of Ireland. Funded under the Health Research Board’s COVID-19 Pandemic Rapid Response Funding Call, this protocol outlines the aims of a project to assess and protect the mental health of the population of Ireland during this pandemic. We will determine (i) the prevalence of common mental health disorders at various times during the first year of the pandemic, (ii) changes in the prevalence of mental health disorders during the first year of the pandemic, (iii) if there are distinct groups of people experiencing different mental health responses to the pandemic, and (iv) the factors associated with different mental health reactions. Methods: This quantitative study uses cross-sectional and longitudinal designs. Data have been collected from a nationally representative sample of Irish adults at four assessments: Wave 1 (N = 1,041) occurred during the first week of lockdown in March 2020, Wave 2 in May, Wave 3 in August, and Wave 4 in December. Wave 5 is planned for March 2021. Participants from Wave 1 have been recontacted at each wave to produce a longitudinal dataset. New participants were recruited using quota sampling to ensure the availability of nationally representative samples at each wave. Self-report measures of demographic, economic, psychological, and mental health variables were completed. Conclusion: This design will allow us to determine whether there has been a change in mental health disorders in the general population during the first year of the pandemic, and if so, what variables are associated with changes in mental health. Results will be used to inform the government’s ongoing response to this crisis, to better protect the mental health of the nation during this and any future public health emergency