Brad Plumer at The New Republic and Nicole Allan at The Atlantic both have useful takes on whether the volcano could lower global temperatures by blocking out the sun. The answer: maybe, if the ash keeps on spewing for a lot longer. They both mention Mount Pinatubo's short-term chilling effect in 1991. My preferred example is Mount Tambora, which blew in 1815 and caused what is known as the Year Without a Summer, frosting the fields of England in July and destroying crops on a wide scale. There were amusing compensating discoveries, though. The lack of livestock feed made it hard to run horse-pulled carriages, so in search of alternatives, Karl Drais started the research that led to the bicycle. And if Joseph Smith's family hadn't fled the crop failures of Vermont and gone to Palmyra, New York, that year, the gold plates and magic spectacles he found in the forest might have lain undisturbed for another 1400 years.
So Eyjafjallajökull may cool the planet, and perhaps indirectly facilitate the excavation of Mormon artifacts. I might point out, though, that some of the concerns about the effects sulfur-aerosol geo-engineering would apply to volcanic-ash cooling as well. Specifically, the cooling is ephemeral, and when the ash-spewing stops, so does the cooling effect. The ash rapidly falls out of the sky. If we have continued to emit carbon during the warming holiday, all the heating the planet would have suffered would hit us all at once, very fast. Instead of adapting over decades to warmer temperatures, we might have to adapt over just a couple years and suffer a sort of climate-change whiplash.