Scottish Gaelic and Irish are closely related. While most dialects are not immediately mutually comprehensible (although many individual words and phrases are), speakers of the two languages can rapidly develop mutual intelligibility.
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Phonetic and grammatical differences
Scottish Gaelic in Islay and Argyll is fairly similar to Irish. The extinct dialects of northeast Ulster, particularly Rathlin Island, were also close to Scottish Gaelic. In Scotland, dialects also existed in southern Kintyre and Galloway which were probably similar to Irish, sliabh (mountain) being fairly common as an element in Galloway and Irish placenames, but quite rare in the Highlands. The dialects of these Scottish and Irish regions are also, in turn, the most similar to 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 long sonants. An example of this is the word clann meaning "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 has a mixed pronunciation, [kɫaunˠ] in the north and [kɫoːdnˠ] in the south. Similarly, im meaning "butter" is pronounced [iːmʲ] in Munster, Southern Connacht and northern Scotland (and Manx), but [imʲ] in Ulster and Southern Scottish.
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 lenits the t, but only after the preverb a "that" (an fear a thá ina sheasamh ag an ndoras "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 Scottish Gaelic and Irish 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 a single friend or family member. In Lewis dialect however, Dè mar a tha thu? is commonly used rather than the polite Dè mar a tha sibh?.
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 Gaelic — 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 [dʲe:g] "died; passed on"). Éa is now used instead of eu in Standard Irish. Éan is written eun in literary Scottish Gaelic. 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 also adapt the spelling eu to how the combination is pronounced in Northern Gaelic, thus ia, writing ian instead of eun. Manx spelling, based mainly on English, shows that ia is also the underlying form in Manx, the word being spelled eean.
Eclipsis
The most obvious phonological difference between Scottish Gaelic and Irish is that the phenomenon of eclipsis in Irish is diachronic (i.e. the result of an historical word-final nasal that may or may not be present in modern Irish) but fully synchronic in Scottish Gaelic (i.e. it requires the actual presence of a word-final nasal except for a tiny set of frozen forms). Eclipsis is shown in the Irish orthography but not in Scottish Gaelic as it is conditioned by the actual environment.
For example, this means that phrases like Standard Irish ag an doras, standard Scottish aig an doras, Manx ec y(n) dorrys is pronounced as follows in different parts of the Gaelic speaking world:
An example of diachronic-type eclipsis are the numbers:
One example of this is the Gaelic "joke" of non-native speakers confusing am bod [əˈmot̪] "the penis" with am mòd [əˈmɔːt̪] "the meeting/get-together" (similar in sentiment to the English speakers' "joke" about non-native speakers confusing "shit" and "sheet").
Orthographic differences
There are a number of distinctive orthographical (written) differences. Both languages have been reformed in recent decades, which has led to further divergence, though conversely more recent spelling reforms in Scottish Gaelic have redressed 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 (Irish: síneadh fada, "length accent"; Scottish Gaelic: stràc gheur, "sharp stroke/accent") used in Irish; hence the word for "welcome" is written as fàilte in Scottish Gaelic and in Irish as fáilte. Irish has no grave accent, only acute accents, while until recently Scottish Gaelic had both grave and acute accents which were used to differentiate between open and closed vowel sounds. However, recent spelling reform has meant that there are now only grave accents in Scottish Gaelic, the opposite of Irish.
The suffix -sa added to the end of a prepositional pronoun for emphasis, is normally hyphenated in Scottish Gaelic, whereas in Standard Irish it is added to the word (though this is frequently not adhered to), as illustrated by the sentence "I've got money":
Scottish Gaelic — Tha airgead agam-sa. Standard Irish — Tá airgead agamsa.Similarly, in Scottish Gaelic the prefix 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 prefixes 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 possible in Classical Gaelic. In the combinations "sc"/"sg" and "st"/"sd", Irish now uses "sc" and "st", while Scottish Gaelic uses "sg" and "sd". In East Ulster, "sd", "chd" and "sg" are still used.
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.