The Sacred Harp style derives from one common to the whole of Britain until the 18th century. It subsequently got stamped out by tidy-minded church reformers everywhere except the Outer Hebrides, but by the time that happened, enough English and Scottish settlers had taken it to the US to keep it viable there.

Pentatonism is no mark of anything. The oldest documented Scottish and Irish music is very firmly heptatonic, and pentatonic modes are found all over the world. If anything pentatonism is often a recent innovation in Scotland and Ireland rather than a relic of anything ancient. One place this comes up is in tunes to be played in medleys, as with dances and marches. Tune-medley dances and military pipe marches are both a 19th century innovation. For both, you need to vary the tonality to keep things interesting, and switching between different gapped-scale modes is an effective way to do that (and on the Highland pipes, about the only effective way). So you suddenly see a whole raft of pentatonic pipe marches coming out of the British Army after 1850, and a similar boom for reels and jigs in the folk-revival era when tune medleys became the rule.

Parallel 4ths and 5ths are pretty rare in any kind of Scottish or Irish tradition, though they were once common in popular religious chants all round Christian Europe.