Background: : In this study, we compared the prevalence of posttraumatic stress disorder (PTSD), depression, and generalized anxiety disorder (GAD) before and during the COVID-19 pandemic across nationally representative samples of Irish adults. Methods: : Participants were sampled in February 2019 (N = 1,020), April 2020 (N = 1,041), May 2020 (N = 1,032), and December 2020 (N = 1,100) using the same self-report measures. Results: : The prevalence of PTSD significantly increased from 12.5% in 2019 to 18.0% in April 2020, to 22.0% in May, and returning to 17.6% in December 2020. PTSD increases were most consistently observed in males, those aged 18–34 years, those without a university qualification, and those living in the Leinster region of Ireland, where the capital city of Dublin is located. There were no significant changes in the prevalence of depression or GAD. Limitations: : The 2020 samples were not completely independent of one another and while the analysis took this into account, this bias cannot be completely removed. Conclusions: : These findings show an increase in PTSD during the initial stages of the COVID-19 pandemic compared to the pre-pandemic period and suggest specificity in mental health responses to the COVID-19 pandemic.