It is thought that Celtic speech rooted itself in Ireland at a very early stage, probably at the end of the Bronze Age, ca. 900 BCE. From the late Neolithic a brilliant native culture had flourished in the area (extending across Ireland, Britain, and the Atlantic seaboard of the Continent), and it is likely that it was in the context of this culture that a distinctive form of Celtic developed, which linguists have called "Goidelic". Other forms of Celtic eventually became dominant in Britain and Armorica, but Goidelic remained the main language of Ireland. Although we have no textual examples of Goidelic from ancient times, we can get an idea of what it was like from the Irish place-names listed in Ptolemy's Atlas, and from the ogham inscriptions found in Ireland and western parts of Britain, mostly dating from the 3rd century CE and a little later. These scanty sources suggest a language still very close to Old Celtic, with fully inflected nouns and verbs.

Although Ireland was never conquered by the Romans (the only part of the Celtic world to evade such conquest), it had regular contacts with the Roman world and its culture, especially by way of Roman-occupied Britain, where many Irish people lived and traded during the last centuries of the Roman Empire. One of the cultural items Ireland imported from the Roman world was Christianity, a religion based on a sacred scripture. Since a full knowledge of the authoritative sources for the new religion required literacy, the ability to use the Latin alphabet became a necessity for the new learned elite studying and teaching in the network of monasteries that began to flourish through out the Celtic world in the late 5th century, and that quickly became the dominant cultural institution in Ireland. At first the only language written and read by monastics was Latin itself, but soon the Latin alphabet was adapted to represent the sounds of the native language of Ireland. Some of the earliest examples of Irish written in the new script are glosses added to religious manuscripts in Latin, clarifying difficult terms for Irish-speakers.

The form of Irish that appears in these texts is called Old Irish (Sean-Ghaeilge), and as a written standard it was used roughly from 700 to 950.  During this period many of the great themes of later Irish literary tradition were first elaborated. Through a fusion of the Christian and pre-Christian educated elites much of the ancient lore that had been passed down by the druidic/bardic schools was adapted to reflect a new social and cultural reality. The earliest versions of the epic _Táin Bó Cualnge_, the earliest recorded stories about Fionn Mac Cumhail, and the first compilations of what would become the new syncretistic history of Ireland, the _Lebor Gabála Érenn_ appear around this time. There is also a body of remarkable nature poetry. Old Irish is characterized by an extremely complex verbal inflection system; its nouns have three grammatical genders (masculine, feminine, and neuter) and five cases.   

After 950 the written standard begins to change in the direction of greater simplicity -- probably reflecting changes in colloquial speech. This stage of the language is called Middle Irish (Meán-Ghaeilge). Most of the great manuscript collections of mediaeval Ireland were compiled during this period -- increasingly the work of secular literary professionals, since the monasteries ceased to be repositories of native learning after the 11th-12th centuries. Some of the texts included in these compilations are clearly much older than the manuscripts themselves, and in some cases Middle Irish "updatings" have been added to them. The rules for the syllabic poetry (_dán díreach_) composed by professional poets also become established by this time.

Around 1350 the language can be said to become Modern Irish (Nua-Ghaeilge). The neuter gender disappeared, and the verbal inflection system was drastically simplified.  As Irish society was brought into the feudal system dominated by an Anglo-Norman upper class, new Continental influences were brought to bear on Irish-language literature, making their appearance in poetry and prose alike. Many of the mythological tales that had been circulating in older versions were given a new form more in tune with the changes that were taking place in the social and cultural climate, focusing on their entertaining qualities rather than on their historical value. Many of the stories about the Fianna -- once considered a "lower class" body of mythology -- were first given literary treatment around this time. Professional poets continued to praise land-rulers -- although those rulers were for the most part no longer of Gaelic stock -- using traditional forms learned in bardic schools.

This world began to collapse when, in the latter part of the 16th century, the English state attempted to consolidate its political power over Ireland by bringing in large numbers of English-speaking settlers to contest and suppress the social and cultural standing of the natives. Throughout the 17th century, despite the tense political situation, Irish remained a prestigious literary language, producing masterpieces like Seathrún Céitinn's _Foras Feasa ar Éirinn_; but after 1695, with the imposition of penal laws on Roman Catholics, English began to gain the upper hand. With the exile or extermination of the educated Irish-speaking upper class, the professional poets no longer had any patrons to support them or any sophisticated audience to appreciate their craft, and those who continued to study in the dwindling bardic schools were forced to abandon the complex forms of _dán díreach_ and adapt to the cruder (in their eyes) accented metres of _amhránaíocht_, characteristic of folksong. By the middle of the 18th century this process was complete. Irish underwent a rapid decline. Even so, a vigorous poetic tradition continued to flourish in certain areas: the poets produced _aislingí_ (vision-poems) where Ireland was personified as an ill-treated woman, or contemporary satire like Brian Merriman's _Cúirt an Mheáin Oíche_.

Although Irish remained the primary language of most of the population of Ireland until the second half of the 19th century, in the absence of formal education in the language a majority of Irish-speakers were no longer able to read or write it. Pockets of Irish literacy remained, especially in Munster; but by and large people were ignorant of the sophisticated literary standard used in 17th century writing and knew only their local vernaculars. The Famine of the late 1840's, which had a particularly severe impact on poorer Irish-speaking areas, further weakened the community standing of the language. However, several of the movements working for the political emancipation of Ireland during the 19th century came to recognize the importance of the language for Irish identity, and soon organizations appeared that promoted the speaking of Irish and encouraged the Anglicized middle class to learn it anew. The founding of the Gaelic League (Conradh na Gaeilge) in 1893 represented the culmination of this process. At first an attempt was made to revive the 17th-century literary language, then to make a nationwide standard of the living dialect of Munster, as the most conservative one and closest to the classical norm. Neither of these approaches were successful in the long run, and the forms of the language actually spoken in the various _Gaeltachtaí_ (Irish-speaking areas) became the basis for what was taught.

As the struggle for Irish independence achieved many of its goals in the course of the 20th century, the Irish language regained some measure of official status and government support. Irish was made a compulsory subject in school (and an Irish proficiency exam was made a prerequisite for coveted civil-service positions), so that practically the entire population of the country was exposed to the language and acquired at least a smattering of it. Unfortunately, this didn't lead to widespread fluency in Irish, much less its re-instatement as the common spoken language throughout Ireland. Middle-class hostility to Irish, strongly established in the 19th century, hasn't been completely reversed; and the position of Irish as a community language remains precarious.

In spite of this, Irish-language literature has had a glorious renaissance in the 20th century, and is still flourishing today. Poetry, novels, short stories, plays, journalism, academic writing -- the full range of literary creativity has been represented, and will amply repay the efforts of anyone who learns the language in order to become acquainted with them.

Irish today is spoken in three main dialects:

-- Munster Irish (Gaeilge na Mumhan, Gaelainn). This is the community language of Corca Dhuibhne on the Dingle peninsula of West Kerry, but it is also spoken by scattered individuals and families in other parts of Kerry, West Cork, and the Ring peninsula of Waterford.

-- Connaught Irish (Gaeilge Chonnachta). This is spoken in Connemara (Co. Galway and southern Co. Mayo) and the Aran Islands.

-- Ulster Irish (Gaeilge Ulaidh). This is spoken in parts of Donegal (Tír Chonaill), especially around Gaoth Dobhair. Although this is a small Gaeltacht, it remains lively and vigorous. This is also the dialect learned by revivalists in nationalist communities in the Six Counties of Northern Ireland.

Each of these dialects presents marked particularisms in pronunciation, grammar, and idioms, but the three are mutually intelligible. Any one of them is appropriate as a beginning point for studying the Irish language. The government has created a _caighdeán oifigiúil_ (Official Standard) – mostly based on Connaught Irish, but incorporating a few features of the other two dialects -- for use in its publications, in the school system, and in the general media, but by and large native speakers remain faithful to their local dialects. Ca. 1950 a major change was introduced into the spelling of Irish, eliminating many "silent" letters; around that time also the Gaelic type that had come into use at the beginning of the revival was, for economic reasons, replaced by ordinary Roman type. Irish orthography remains almost completely phonetic, which is a great help in learning the language.