The changes to the Irish exclusionary rule introduced by the judgment in People (DPP) v JC mark an important watershed in the Irish law of evidence and Irish legal culture more generally. The case relaxed the exclusionary rule established in People (DPP) v Kenny, one of the strictest in the common law world, by creating an exception based on ‘inadvertence’. This paper examines the decision through the lens of legal culture, drawing in particular on Lawrence Friedman’s distinction between ‘internal’ and ‘external’ legal culture to help understand the factors contributing to the decision. The paper argues that Friedman’s concept and, in particular, the dialectic between internal and external legal culture, holds much utility at a micro as well as macro level, in interrogating the cultural logics at work in judicial decision-making.