Jonnie Robinson, Curator, English accents and dialects, British Library Sound Archive, writes:
Code-switching - mixing words, phrases or even whole sentences from two different languages within the same conversation - is typical of speakers in bilingual communities. In most cases this process is subconscious and, as these speakers explain, it indicates that a speaker feels a particular word expresses far more more accurately the meaning they're trying to convey, such as the words given here: dreach, meaning aspect, mosach meaning nasty or dirty and smurach meaning drizzle. The occasional or even frequent use of a Gaelic word or expression within an English sentence can communicate a great sense of shared identity or solidarity with other speakers.
Equally importantly perhaps in terms of the nature of English spoken in Scotland, listen to the 'guttural' sound used for the final consonant in all these words: dreach, mosach and smurach. This is a sound produced in the velar region at the back of the mouth and used, for instance, in the traditional Scottish pronunciation of the word loch and a sound we might perhaps associate with spoken German. English is of course a Germanic language and at one time this sound was in fact a feature of spoken English throughout the UK and explains some of our seemingly anomalous spellings, such as night, through and cough, where the would originally have been pronounced using a similar sound. In most cases, such as night and through (modern German Nacht and durch) this sound has now disappeared, whereas in other cases, such as cough it has transmuted to an sound. The velar articulation is, however, part of the sound system of Scottish and Irish Gaelic and thus it's not surprising that it has even been retained in a number of words in the English spoken in those two areas.