In reply to aln:
It would have been the first of the two I said in my last post. It'd be easier to explain with a diagram but not sure that UKC has a drawing board function?! That would be good for this thread!
But yeah it is probably a slightly unusual situation. I'd explain it as follows:
The trench that holds Loch Maree is just a big valley carved out by a large glacier (several times over during successive glaciations) and the land that represents the Islands is just some rock which is for some reason more resistant to erosion (perhaps different lithology, or maybe just very well cemented). But for whatever reason, the ice has gone around it rather than completely removing it.
Then when the last ice melted and Loch Maree was left, (perhaps with a higher water level than the present) the 'islands' of resistant rock have been drowned to varying degrees. The small Lochan on the Island is just some pre-existing hollow in the land which happens to be deeper than the current Loch Level at the moment. Its probably been left full of water when Loch Maree had a higher water level and drowned the whole Island. Then when the Loch level lowered (for whatever reason) the Islands were exposed, and any hollows left full of water.. that's how that wee Lochan is there.
But you're probably right, I don't know that it is a particularly common situation. Nae sure! But the above is how I would attempt to explain it.
Hope this helps...