There are a number of distinct dialects of Irish. Roughly speaking, the three major dialect areas coincide with the provinces of Munster (Cúige Mumhan), Connacht (Cúige Chonnacht) and Ulster (Cúige Uladh).

Munster dialects
Munster Irish is spoken in the Gaeltachtaí of Kerry (Contae Chiarraí), Muskerry (Múscraí), Cape Clear (Oileán Cléire) in the western part of County Cork (Contae Chorcaí), and the tiny pocket of Irish-speakers in An Rinn near Dungarvan (Dún Garbháin) in County Waterford (Contae Phort Láirge). The most important subdivision in Munster is that between Decies Irish (spoken in Waterford) and the rest of Munster Irish.

Some typical features of Munster Irish are:
The use of personal endings instead of pronouns with verbs, thus "I must" is in Munster caithfead, while other dialects prefer caithfidh mé (mé means "I"). "I was and you were" is "Bhíos agus bhís" in Munster but "Bhí mé agus bhí tú" in other dialects.
In front of nasals and "ll" some short vowels are lengthened while other are diphthongised.
A copula-construction involving is "ea" is frequently used.

Connacht dialects:
The strongest dialect of Connacht Irish is to be found in Connemara and the Aran Islands. In some regards this dialect is quite different from general Connacht Irish but since most Connacht dialects have died out during the last century Connemara Irish is sometimes seen as Connacht Irish. Much closer to the traditional Connacht Irish is the very threatened dialect spoken in the region on the border between Galway (Gaillimh) and Mayo (Maigh Eo). The Irish of Tourmakeady (Tuar Mhic Éadaigh) in southern Mayo (Maigh Eo Theas) and Joyce Country (Dúthaigh Sheoige) are considered the living Irish dialects closest to Middle Irish. The northern Mayo dialect of Erris (Iorras) and Achill (Acaill) is in grammar and word-building essentially a Connacht dialect, but shows an affinity in vocabulary with Ulster Irish, due to large-scale immigration of dispossessed people following the Ulster Plantation.
Connemara Irish is very popular with learners, thanks to Mícheál Ó Siadhail's self-teaching textbook "Learning Irish". However, there are features in Connemara Irish which are not accepted standard, notably the preference for verbal nouns ending in -achan, such as lagachan instead of lagú, "weakening". The non-standard pronunciation with lengthened vowels and heavily reduced endings give Connemara Irish its distinct sound.

Ulster dialects:
The most important of the Ulster dialects today is that of the Rosses (na Rosa), which has been used extensively in literature by such authors as the brothers Séamus and Seosamh Mac Grianna, locally known as Jimí Fheilimí and Joe Fheilimí. This dialect is essentially the same as that in Gweedore (Gaoth Dobhair= Inlet of Streaming Water), the same dialect used by native speaker Enya (Eithne) and her siblings in Clannad (Clann as Dobhar = Family from the Water).
Ulster Irish sounds very different and shares several unusual features with Scottish Gaelic, as well as having lots of characteristic words and shades of meanings. However, since the demise of those Irish dialects spoken natively in what is today Northern Ireland, it is probably exaggerated to see Ulster Irish as an intermediary form between Scottish Gaelic and the southern and western dialects of Irish. Indeed, Scottish Gaelic does have lots of non-Ulster features in common with Munster Irish, too.

One noticeable trait of Ulster Irish is the use of the negative participle "cha(n)", in place of the Munster and Connaught version "ní". Even in Ulster, cha(n), most typical of Scottish Gaelic, has ousted the more common ní only in easternmost dialects (including the now defunct ones once spoken in what is now Northern Ireland). The practice seems to be that cha(n) is most usually used when answering to a statement, either confirming a negative statement (Níl aon mhaith ann - Chan fhuil, leoga = "It is no good" - "Indeed it isn't") or contesting an affirmative one (Tá sé go maith - Chan fhuil! = "It is good" - "No, it isn't!"), while ní is preferred in answering a question (An bhfuil aon mhaith ann? - Níl = "Is it any good?" - "No").

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Other regions
The extant dialects of Irish native to Leinster, the fourth province of Ireland, became extinct during the 20th century, but records of some of these were made by the Irish Folklore Commission among other bodies prior to this.
The Irish of Meath (in Leinster) is a special case. It belongs to the Connemara dialect, as the Irish-speaking community in Meath is simply a group of mostly Connemara speakers who moved there in the 1930s, after a land reform campaign spearheaded by Máirtín Ó Cadhain, subsequently one of the greatest modernist writers in the language.

Comparisons
The differences between dialects are considerable, and have led to recurrent difficulties in defining standard Irish. Even everyday phrases can show startling dialectal variation: the standard example is "How are you?":
Ulster: cad é mar atá tú? ("what is it as you are?" Note: caidé or goidé are alternative renderings of cad é)
Connacht: cén chaoi a bhfuil tú? ("what way [is it] that you are?")
Munster: conas taoí? ("how are you?")
In recent times, however, contacts between speakers of different dialects have become more common, and mixed dialects have originated. Nevertheless, many dialect speakers (especially Ulster) are still jealously trying to guard their own variety against influences from other dialects. Among non-native speakers, this can be seen as a quest for authenticity. Regional accents are commonly taught to non-natives and imitated: an urban non-native speaker of Irish in Cork City (Cathair Chorcaí) is very probably trying to emulate Coolea or Kerry dialect; one from Belfast (Béal Feirste) tends to speak an Irish modelled on the Rosses dialect of Donegal; and Galwegian Irish-speakers, living next door to Connemara, will do their best to sound like a Connemara native.

There also exists a cant called Shelta, based partly on English and partly Irish, in use by the Irish Travellers.