In simple terms, with regards to Scottish Gaelic, pre-aspiration is essentially the equivalent of a [h], [x] or [ç] sound before a [t], [k] or [p]. Apart from it's presence in Scottish Gaelic, pre-aspiration is most often associated with North Germanic languages. In particular, it is associated with the Icelandic and Faroese languages, but in addition to this, it can also be heard in some dialects of Norwegian and Swedish.
One theory of it's presence in Scottish Gaelic is that it is an example of the Norse influence on the Gaelic language of Scotland. The presence of pre-aspiration makes Scottish Gaelic unique among her sister languages, Irish and Manx, where no evidence of pre-aspiration is to be found in any dialect. In addition to this, the patterns of pre-aspiration vary from dialect to dialect with some dialects showing no evidence of pre-aspiration at all.
There are five patterns of pre-aspiration to be found within the study area, detailed as follows:
Pattern 1: / h / before / t / and / p / but no sound before / k /
Pattern 2: / x / or / ç / before / t /, / p / and / k /
Pattern 3: No sound before / t / or / p / but / x / before / k /
Pattern 4: No pre-aspiration
Pattern 5: / x / before / t /, / h / before / p / and / x / before / k /
Pattern 2: / x / or / ç / before / t /, / p / and / k /
Pattern 3: No sound before / t / or / p / but / x / before / k /
Pattern 4: No pre-aspiration
Pattern 5: / x / before / t /, / h / before / p / and / x / before / k /
Pattern 1 - This pattern was to be found largely in the islands in Coll, Tiree, Colonsay and Jura as well as parts of Islay, Mull. In addition, isolated occurances of this pattern were recorded on the mainland in Ardnamurchan, Stronmilchan, Craignish and Tayvallich.
Pattern 2: This pattern was recorded in North Argyll, including the islands of Luing, Lismore and East Mull. One isolated occurrence was recorded in Bowmore on Islay.
Pattern 3 - This was the pattern recorded in Mid-Argyll, with the exceptions of Tayvallich and Craignish. In addition, the pattern was also recorded in Gigha and the Rinns of Islay as well as one isolated occurrence in Kintyre.
Pattern 4: This was the pattern recorded in all areas of East Ulster and Mann. In addition to this, it was the pattern recorded in Arran and in most parts of Kintyre.
Pattern 5: Only one occurrence of this pattern was recorded in Morvern.
It is clear to see from the isogloss maps that the dialects of Carrick and Galloway would have fallen into line with the neighbouring dialects of Kintyre and Arran, as well as those of East Ulster and Mann. As well as making a fairly educated guess based on geographically proximity, we can also look to place-names for further clues.
Girvan MacKay points to two examples of the local pronunicaiton of placenames: Beinn Bhreac (Benbrack) and Achadh nam Breac (Auchenbrack). The local "Anglicised" pronunciation gives us an indication that the pre-aspiration was absent from the dialects of Carrick and Galloway.
Had pre-aspiration been an aspect in these dialects then it certainly would have made it an outlier among the surrounding forms of the Gaelic tongue.