aːRd vs aːRʃd or Where the sh comes from
We won't go into the discussion of whether this feature is Norse or Martian because unless you are hoping to become a historical linguist, it doesn't matter.
So what *can* we tell you about this strange [ʃ] that pops up in Gaelic?
It affects rt and rd groups in Gaelic.
Traditionally, not all dialects are affected, as you can see on the below map and Perthshire Gaelic even diphthongised the [a]. Due to the pervasiveness of Lewis Gaelic in the media, teaching etc, this feature has spread into other dialects, so the presence or lack of the [ʃ] is no clear cue to geographical origin anymore.
If it occurs, it tends to occur only in stressed syllables so ainneart for example tends not to have this intrusive [ʃ].
Some speakers tend to insert [ʃ] only in rt groups, but not rd groups, but never the other way round. Thus you will hear speakers who pronounce ceart and àrd as [kʲaɹʃt̪] and [aːɹʃt̪] but *never* [kʲaɹt̪] and [aːɹʃt̪].
Beyond that, there doesn't appear to be a clear rule, but we'll research this further in the future and see whether there actually is a rule.
It seems to be a good guideline to insert [ʃ] both in rt and rd groups in stressed syllables (i.e. the first) as a learner - it won't sound wrong to a Gael's ears even should he not pronounce it himself. Unless you are desperate to learn a particular dialect (which we don't recommend anyway), in which case you will have to memorise your dialects rt and rd words, as it does not appear to be rule based as far as we can see.
To listen to some examples, check our Fuaimean na Gàidhlig section.
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