A new species of slug, Arion occultus, is described from Co Down in Northern Ireland. It belongs to the hortensis group in subgenus Kobeltia of Arion, and is superficially similar to Arion distinctus Mabille. Its origins are unknown and it appears to be a recent immigrant to Ireland. Distinctive features include: a rough external appearance due to strongly raised and keeled tubercles on the dorsum; a uniformly yellow and rather broad foot fringe; an epiphallus which lacks either pigmentation or a bulbous swelling at its apex and which also lacks an epiphallus process at the junction of the epiphallus with the distal oviduct, features which are otherwise general in the hortensis group; the possession of a simple spermatophore comprising an oblong, flattened sac lacking serrations or other distinctive features. Externally the integument possesses abundant yellow chromatophores lightening an otherwise predominantly dark dorsum. Size is larger than distinctus with broad, high lateral bands, below which the sides are predominantly pale or white, rather than grey. Although superficially similar to the central European Arion alpinus the latter does not belong to the hortensis group and has a very different genital anatomy.