Is it weird that the Quiet Isle has apparently been unmolested for many centuries? Also that – afawk – Dorne & the major Blackwater Bay isles (pre-Valyrian colonisation) are & were immune to being the targets or haunts of pirates, despite being rather prime candidates (Planky Town anyway).

Actually, there’s some very good reasons:

1. The Quiet Isle is not on the west coast, so it doesn’t have to deal with Ironborn Lindisfarne re-enactors. 
2. The Quiet Isle is pretty far up the Trident, so it’s a long way for Narrow Sea pirates to sail, compared to preying on the major trade routes.
3. The Quiet Isle is on incredibly shallow waters (which is a bit odd, you would think that the confluence of the Trident would result in a fairly deep channel, but it must be a very wide and shallow bay), so that it’s actually landlocked when the tide goes out. Thus, there’s daily periods when pirate ships would find themselves beached on the way out or running aground on the way in.