Honestly surviving volcanic eruptions on Iceland aren’t that hard, yes the aftermath decimate the population, as it poison the land based food chain, but a significant part of the population will almost certainly survive.
A interesting aspect of this settlement are founder effect. The Ireland and Scotland are known fair having the highest percent of redheads, a mix of founder effect, selection (red hair likely served as alternative to the selection toward blond hair we saw around the Baltic among agricultural people[1]), small population and pure chance could result in the population being fully red haired.
[1] blond and red hair are connected to pale skin, which are a benefit for agricultural people at high latitude.