Background and Objectives: Loneliness and existential loneliness are undesirable states which may have been inflated during the COVID-19 pandemic across the population. The Objective was to assess trajectory of loneliness among Irish adults during the COVID-19 pandemic, predictors of baseline and changes in loneliness over time, and to evaluate existential loneliness towards the end of the pandemic, and its correlates. Methods: Longitudinal panel data were gathered from 1041 participants aged 18-80 from 31st March 2020 across four waves. Latent growth models in a structural equation modelling context were used to evaluate change in loneliness over time. A separate structural equation model was used to evaluate correlates of existential loneliness. Results: At a group level there was little change in loneliness over time (estimate = 0.04) and participants were on average moderately lonely at baseline (intercept estimate = 1.62). At baseline loneliness was associated with: being younger, not being in a relationship, being employed in healthcare, lower levels of neighbourhood belongingness, social contact from family, and higher levels of depression. Change in loneliness was associated with: being employed in healthcare (negative change, possibly regression to mean), and depression levels (negative change, again likely regression to mean). Existential loneliness was associated, cross-sectionally, with loneliness (beta = .46), depression (beta = .31), sex (beta = -.134), age (beta = -.19), relationship status (beta = .14), and empathy (beta = -151). Conclusions: We describe correlates of loneliness in the first Irish lockdown of 2020, as well as predictors of changes in loneliness across the pandemic, and correlates of existential loneliness at a later stage in the pandemic. Results indicate that those working in healthcare in particular had highest levels of loneliness during the first lockdown. Results are discussed in the context of potentially informative theoretical frameworks of loneliness.