Although Irish, Manx and Scottish Gaelic are closely related as Goidelic (a.k.a. Gaelic) Celtic languages, they are different in many ways. While most dialects are not immediately mutually comprehensible (although many individual words and phrases are), speakers of the three languages can rapidly develop mutual intelligibility.^{[citation needed]}

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The spoken dialects of Irish and Scottish Gaelic are most similar to one another in Ulster and southwestern Scotland, regions of close geographical proximity to one another. It is thought that the extinct dialect of Galwegian Gaelic, spoken in Galloway in the far south of Scotland, was very similar to Ulster Irish and Manx.

While the dialects of northern Scotland and southern Ireland tend to differ the most from one another in terms of vocabulary, they do share some features which are absent in other dialect areas lying between them. For example, in both Munster Irish and the Gaelic of the north of Scotland, historically short vowels have been diphthongised or lengthened before fortis sonorants. An example of this is the word clann "children of the family". In Munster Irish and northern Scottish Gaelic it is pronounced [kɫaunˠ] whereas in Ulster and County Mayo it is [kɫanˠ] and in Connemara [kɫɑːnˠ]; the Manx form cloan is [kɫaunˠ] in the north and [kɫoːdnˠ] in the south.

In addition, slender coronal stops (/tʲ/ and /tʲʰ/ in Scottish Gaelic; /tʲ/ and /dʲ/ in Irish), are affricated (such as [tʃ] and [dʒ]) in Mayo^[1][2] and Donegal,^[3] the southern Highlands and in Manx, but not in Munster or the northern Highlands.^[4]

In the verb tá of Standard Irish, northern Scotland and Central-Southern Munster agree in leniting the initial ⟨t⟩, thus one hears thá in County Waterford and County Tipperary, and tha in northern Scotland. West Munster also lenites the ⟨t⟩, but only after the preverb a "that" e.g. an fear a thá ina sheasamh ag an doras "the man that's standing at the door" (Standard Irish an fear atá ina sheasamh ag an doras, Scottish Gaelic am fear a tha na sheasamh aig an doras).

The closest to Scottish Gaelic in modern Irish is the dialect currently spoken in County Donegal, as illustrated by the sentence "How are you?".

Scottish Gaelic: Ciamar a tha sibh? (plural/formal) or Ciamar a tha thu? (singular/informal), Lewis dialect Dè mar a tha sibh? (plural/formal) Dè mar a tha thu? (singular/informal) (dè < cad è)

Ulster Irish: Cad é mar atá sibh? (plural) Cad é mar atá tú? (singular), spelt in 'dialect spelling' as Caidé mar a tá sibh/tú?

Connacht Irish: Cén chaoi a bhfuil sibh? (plural), Cén chaoi a bhfuil tú? (singular), in colloquial speech Ce chuil sib/tú

Munster Irish: Conas táthaoi (plural), Conas taoi? (singular), Conas tánn sibh/tú?, Conas atá sibh/tú?

Sibh is used in both Irish and Scottish Gaelic for the plural "you", while Scottish Gaelic (except for the far south) also uses sibh as a formal version of "you" (much like French uses vous; see "T–V distinction"). Modern Irish does not use this formal/informal distinction when addressing people. The use of sibh as 'polite' you is a retention from the Classical Irish usage of the plural personal pronouns to refer to the singular in polite communication, thus sinn "we" for mé "I, me" and sibh "you (plural)" for tú "you/thou". Thu is used in Scottish Gaelic when speaking to an individual friend, family member, or a younger person.

The negative particle in Scottish Gaelic, Manx and Northern Ulster Irish is cha/chan (chan eil, cha bhfuil/chan fhuil = "is not"; chan is from the Old Irish emphatic negative nichon). In standard Irish the negative particle is ní (níl = "is not", a contraction of ní fhuil); ní is a retention of the normal Old Irish negative; these are illustrated by the sentence "I have no money":

Scottish Gaelic: Chan eil airgead agam.

Ulster Irish: Chan fhuil/Cha bhfuil airgead agam.

Manx: Cha nel argid aym.

Standard Irish: Níl airgead agam.

Scottish Gaelic speakers may also sound as if they were using the Irish phrase, as Chan eil can frequently be shortened to n eil.

The Classical Irish digraph ⟨éu⟩ [eːʷ] is still used in Scottish Gaelic spelling but is now obsolete in Irish, except in southern dialect writing, as a means to distinguish the vowel ⟨é⟩ when followed by a broad consonant from the regular dialect development ⟨é⟩ to ⟨i⟩ in the same environment, thus éan [ian] "bird" in comparison to d'éug [dʲeːɡ] "died; passed on). ⟨éa⟩ is now used instead of ⟨eu⟩ in Standard Irish. Both ⟨éa⟩ and ⟨éu⟩ existed in Classical Irish, to a large extent showing nominal case differences (with ⟨éu⟩ varying with ⟨éo⟩ in the dative of ⟨éa⟩-words), however in both Scotland and Ireland, spelling reforms and standardisation (which took place in Ireland under the auspices of the government of Ireland during the 20th century, and much earlier in Scotland) independently went for different versions.

At times Scottish writers used the spelling ⟨ia⟩ to represent how the combination is pronounced in northern dialects, writing ⟨ian⟩ instead of ⟨eun⟩, the southern form. Manx spelling, based mainly on English, shows that ⟨ia⟩ is also the underlying form in Manx, the word being spelled ⟨eean⟩.

There are a number of distinctive orthographical (written) differences. The spellings of both languages have been reformed in recent decades, which has led to further divergence, though conversely more recent spelling reforms in Scottish Gaelic have reduced the divergences to some extent.

One difference is that the accent is written as a grave accent (Scottish Gaelic: stràc throm, "heavy stroke/accent") in Scottish Gaelic, as opposed to the acute accent ((síneadh) fada, "long (sign)" used in Irish; hence the word for "welcome" is written as fàilte in Scottish Gaelic and in Irish as fáilte. Irish does not use the grave accent, while until recently Scottish Gaelic used the grave and acute accents to differentiate between open and closed vowel sounds. However, recent spelling reform has meant that only grave accents are now in Scottish Gaelic, leaving phonemic distinctions unmarked.

Another difference in Scottish Gaelic is that the aspirate linker h- is always hyphenated, while in Irish it is attached to the beginning of the word, as illustrated by the languages' respective names for each other:

Scottish Gaelic — Gàidhlig (na h-Alba), Gàidhlig na h-Èireann

Standard Irish — Gaeilge na hAlban, Gaeilge (na hÉireann)

Additionally, while the linkers n- and t- are usually hyphenated in both languages, in Irish they are attached to the beginning of words whose first letter is capitalised; in Scottish Gaelic they are always hyphenated.

A number of letter combinations are possible in written Irish which are not found in Scottish Gaelic e.g. ⟨ae⟩, ⟨bhf⟩. Irish uses ⟨cht⟩ where Scottish Gaelic uses ⟨chd⟩, although ⟨chd⟩ itself was once common in written Irish, as was ⟨cht⟩ in Scottish Gaelic – both being used in Classical Gaelic. In the combinations ⟨sc/sg⟩ and ⟨st/sd⟩, Irish now uses ⟨sc⟩ and ⟨st⟩, while Scottish Gaelic uses ⟨sg⟩ and both ⟨sd⟩ and ⟨st⟩, despite there being no phonetic difference between the two languages.^[7]

Most obvious differences in spelling result from the deletion of silent lenited digraphs (mainly ⟨dh⟩, ⟨gh⟩, and ⟨th⟩) in Irish in spelling reforms, which was only sometimes done in Scottish Gaelic. Overall, Scottish Gaelic orthography is more conservative than that of Irish.

Differences can also be seen in words used for geographical features. For example, "hill" and "mountain" are usually "cnoc" (Knocknapeasta) and "sliabh" (Slieve Donard) respectively in Ireland, but "càrn" (Cairn Gorm) and "beinn" (Ben Nevis) in Scotland. Additionally, "inbhir," meaning "river mouth" and usually Anglicized as "inver" (for example Inverness or Inveraray), very common in Scotland, is almost never seen in Ireland.

Article 1 of the UDHR in the languages:

• An Caighdeán Oifigiúil (standard of orthography and grammar used by the Government of Ireland)
• Dialect continuum
• Early Modern Irish
• Middle Irish
• Old Irish