Just a question of curiosity. I understand that /ʃ / being found in the ‘rt’ grouping is the norm, but that its presence or absence in ‘rd’ groupings varies by dialect (or so a fair bit of instructional material has pointed out). My question is, how common is it? I'm wondering if either the presence or the absence is a more niche variant, or if there's a pretty even distribution. So far, all of the audio examples I've heard have included this, so my impression is that it's more common than not, but I'm really lacking the experience to make any sort of assessment of my own.

Only a few dialects in the southwest of Scotland (Arran, Kintyre...) do not pronounce rt as st, rsht etc, and these dialects have very few speakers nowadays, in my opinion, no wonder you don't hear these dialects in the Gaelic media. Most of what you can hear is Gaelic from the Outer Hebrides, that most native speakers come from, and in these dialects, they do pronounce rt as rsht etc...

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

Apologies, I think maybe I didn't specify clearly enough in my question - I mean specifically how common the presence vs. absence is specifically in the ‘rd’ cluster. I've taken the ‘rt’ as given, but sources seem to point out a difference in the ‘rd’ grouping specifically. Does that change anything?

In the cluster "rd" it's not as common.
You hear a sh or an s sound in the following areas : Harris, North Uist, Coll, Colonsay, in the southwest of the Highlands, in parts of Skye, and in a few places in the western Highlands. Source: Survey of the Gaelic Dialects of Scotland (vol. II, with the word "àrd").

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

Perfect - source and all! Thanks