The COVID-19 Psychological Research Consortium (C19PRC) study was established to determine the impact of the COVID-19 pandemic on the population of multiple countries. Here, we provide a methodological overview, cohort profile, data access, and summary of key findings from the Republic of Ireland arm of the C19PRC study. A longitudinal internet panel survey was designed to collect data from a nationally representative sample of Irish adults ( N = 1041) who were tracked from March/April 2020 to March/April 2021. Quota sampling methods were used to produce a sample that was representative of the population with respect to sex, age, and regional distribution. Data were collected in five waves, and new participants were recruited at follow-up waves to cover sample attrition and produce nationally representative samples at various points during the first year of the pandemic. A comprehensive battery of measures was used throughout the project to assess an array of sociodemographic, political, social, psychological, physical health, COVID-19, and mental health variables. Analyses were conducted to compare sample characteristic to known population parameters from available census data. These analyses showed that the sample was representative of the general adult population of Ireland on the three quota variables and was reasonable representative of the population across a diverse range of sociodemographic variables. These data representative the first and only nationally representative, longitudinal survey of the mental health of the Irish population. These data are made freely available to interested users ( https://osf.io/2huzd/files/ ) and the findings of this study provide a methodological basis for the future use of these data.