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Ireland, being an island situated on Europe's western seaboard, has a fewer number of native species than mainland European Union Member States (MS). Increased numbers of vectors and pathways have reduced the island's biotic isolation, increasing the risk of new introductions and their associated impacts on native biodiversity. It is likely that these risks are greater here than they are in continental MSs, where the native biodiversity is richer. A horizon scanning approach was used to identify the most likely invasive alien species (IAS) (with the potential to impact biodiversity) to arrive on the island of Ireland within the next ten years. To achieve this, we used a consensus-based approach, whereby expert opinion and discussion groups were utilised to establish and rank a list of 40 species of the most likely terrestrial, freshwater and marine IAS to arrive on the island of Ireland within the decade 2017-2027. The list of 40 included 18 freshwater, 15 terrestrial and seven marine IAS. Crustacean species (freshwater and marine) were taxonomically dominant (11 out of 40); this reflects their multiple pathways of introduction, their ability to act as ecosystem engineers and their resulting high impacts on biodiversity. Freshwater species dominated the top ten IAS (seven species out of ten), with the signal crayfish (Pacifastacus leniusculus) highlighted as the most likely species to arrive and establish in freshwaters, while roe deer (Capreolus capreolus) (second) and the warm-water barnacle (Hesperibalanus fallax) (fifth), were the most likely terrestrial and marine invaders. This evidence-based list provides important information to the relevant statutory agencies in both the Republic of Ireland and Northern Ireland to prioritise the prevention of the most likely invaders and aid in compliance with legislation, in particular the EU Regulation on Invasive Alien Species (EU 1143/2014). Targeted biosecurity in both jurisdictions is urgently required in order to manage the pathways and vectors of arrival, and is vital to maintaining native biodiversity on the island of Ireland. [ABSTRACT FROM AUTHOR]