The focus of this chapter is (i) the nineteenth century as the formative period for Irish English as a high-contact variety of English, and (ii) increase in letteracy among the lower classes, though not necessarily literacy, which led to the development of a written vernacular. Many typical features, e.g. the after-perfect, second person plural pronoun yous, subordinating and, and habitual do be, are claimed to have originated or at least developed its current form or function in the nineteenth century. The language shift from Irish to English, though starting much earlier, gathered momentum in the nineteenth century due to unguided, rapid acquisition of English. This period saw a continuation of older features of Irish English as well as a high number of new developments, both contact-induced and independent developments as a result of ‘imperfect learning’ by broad sections of the population.