Volcanic aerosols in the stratosphere has a cooling effect, but lasts only a few years at most before they fell down, while ice sheet melting and distabilization from volcano eruptions below and above the ice sheet, as well as the damage to the ozone hole, if it's a large eruption or simultaneous eruption of several volcanoes, would have a more long term effect.

Volcanic ash falling on the ice surface also has a strong warming effect by reducing the surface albedo.

Maggie