It is well recognised that there is an intimate relationship between sleep and depression, with poor quality or short duration sleep associated with greater symptoms of depression. However, it is not clear from the current evidence base what the temporal relationship is between symptoms of insomnia and depression. Further, it is also unclear how the COVID-19 pandemic may impact on such relationships. In this study we have examined the longitudinal relationships between symptoms of depression and insomnia during the COVID-19 pandemic at two points separated by one year (April/May 2020 and March/April 2021) in a sample of 1032 Irish adults using a cross-lagged paths model. We report that there is a bidirectional relationship across time between depression and insomnia symptoms (β = -0.115 between Insomnia symptoms and subsequent depression symptoms and β = -0.163 between depression symptoms and subsequent insomnia symptoms; scales scored in opposite directions), and that these relationships persist when COVID-19 anxiety, age and sex are introduced into the model. Our analyses suggest that during the first year of the COVID-19 pandemic that insomnia symptoms predicted depression symptoms one year later, and conversely that depression symptoms predicted subsequent insomnia symptoms.