Old Irish was the language of Ireland’s “Golden Age”, and its classical phase is generally assigned to the period AD 700-850. Old Irish evolved into Middle Irish, the language of the late Viking and post-Viking period. In comparison with Old Irish, Middle Irish is characterized by a simplification of the inflections of noun and verb and of the system of pronouns.

The earliest known form of Irish is preserved in Ogham (or ogam) inscriptions that date from the fourth and fifth centuries. The linguistic information preserved in Ogham is sparse, as the inscriptions contain little more than personal names, but it is sufficient to reveal a form of Goidelic much older than Old Irish, the earliest documented variety.

By 1200 Early Modern Irish, or Classical Modern Irish, had begun to emerge. This is the language of the period of Gaelic resurgence when Old Irish, Norse, Norman, and Old English were largely assimilated into a new Irish-speaking society. This form of Irish lasted from the thirteenth century to the seventeenth as the literary norm for the whole Gaelic world, which comprised Ireland, Gaelic Scotland, and the Isle of Man.

During the seventeenth century, as the influence of the old literary schools and learned classes receded, the forms of the written language became increasingly regional in character. In this period the autonomous forms of Modern Irish, Scots Gaelic, and Manx became established. Even so, at the end of the nineteenth century and the beginning of the twentieth, as the Irish revival gathered momentum, there were many who felt that Classical Modern Irish was still the most appropriate norm for literary purposes. Since the advocates of this view not only used the older grammatical forms, but also imitated the ornate and sometimes ponderous style of Early Modern prose, they brought a reaction from writers such as Peadar Ó Laoghaire, Pádraic Ó Conaire, and Patrick Pearse, who were developing a literary diction based on contemporary speech.

The “speech of the people” movement triumphed but one result was that the written language for a time became quite diversified, as writers went their divergent ways in representing contemporary usage. It was necessary to redefine norms and a new spelling norm was published in 1945 with several later revisions. Modern Irish, whilst always changing, is codified in the official Irish-English dictionary, Foclóir Gaeilge-Béarla, which first appeared in 1978.