ciarili - at a guess it was around 1159 BC, the time of the Hekla III eruption in Iceland. This could well have been what led to a dust cloud which covered the northern hemisphere and was recorded as far away as China.

By any chance, was the dendrochronolgist Mike Bailie (a palaeoecologist from Queen's University, Belfast)? Big on comet theories? He has us all panicking in a lecture one day when he talked about super volcanoes and the fact that if the whole of the earth is wrapped in a dust cloud, then there's only enough food to last something like 42 days if evenly distributed, as food production isn't possible under such conditions. He described the scenario in scary, vivid detail, and then finished by saying "now that's what they don't teach you about volcanoes in school".

As an aside, there is scant evidence to date for palaeolithic (old stone age) settlement of Ireland with the exception of a worked flint which was probably carried there by glaciation. The nice thing about archaeology, however, is that our view of the past is only temporary, and changes when new information comes to light.