A lot of the Outer Hebrides Islands look quite similar to the Moors in the South West, Dartmoor and Bodmin Moor, and what is noticeable is the lack of trees, we do know that the Moors and most of the country was once dense forest, and we can assume that the trees were used as fossil fuels, it would be interesting to know, once the trees were gone did the inhabitants turn to burning the peat, and if they did, then they could well have depleted the supply of peat quite quickly, did they then move on to other areas where there was a good supply of trees or peat.
Regarding lighting fires and the methods used, I would assume flint would be the main method, and I also think that fires were never allowed to go out, this would mean that small settlements would be moved to areas of plentiful supply, in order to ensure a good supply of fuel , whether it was trees or peat. The Hebrides are covered in small lakes and indentations, maybe these were the original areas of extraction.