Confidence: I'm as sure as I can be.
Notes: The finger-print ridges on the mantle (flap of skin behind the head) make it Limacidae. The blue-grey tentacles make it Limacus. The large pale blotches on a variable but mostly khaki background, and the lack of a pale stripe along the keel (faint ridge down the middle of the back) all suggest Irish Yellow Slug L. maculatus rather than Yellow Slug L. flavus. It's quite widespread in Britain, and often found in gardens.