The XBeach model has been used to simulate the morphological impacts of storms on sandy and gravel beaches. Taking as a case study Rossbeigh Spit located on the high-energy coast of western Ireland, the study reported here tests the capacity of XBeach to reproduce barrier breaching during a storm in December 2008. It demonstrates that predictions of the breaching event agree reasonably well with observations. However, the main focus of the paper is to establish using the model results, sitespecific critical wave and water level conditions giving rise to dune erosion, overwashing and breaching. By deriving simple-to-use expressions to define hydrodynamic thresholds the study advances the ability to predict the impacts of infrequent and rarely observed storm events and is considered to provide useful coastal management tool for assessing the vulnerability of sandy barriers to breaching high-energy during storms.