There is a constant background level of volcanic activity every year. The Smithsonian Museum has kept detailed records since 1960. It shows that there are on average 50 - 70 volcanic eruptions each year and the trend line is more or less horizontal suggesting that the effect of volcanoes on global warning is neutral and unchanged over that period.
The Smithsonian also publish a list of major volcanic eruptions since roughly 10,000 BC. This is a list of volcanic eruptions scoring above 4 on a scale that goes from 1 - 6 (that was the highest number I could find in this 12,000 year sequence). Level 4 eruptions happen every 1-2 years, that unpronounceable, unspellable Icelandic volacano was a Level 4 eruption. Mt St Helen(1984) was a Level 5, and these occur roughly every 50 years. Level 6 eruptions like Krakatoa(1883) occur roughly every century or less.
I am not aware that the Icelandic volcano had any major effect on either micro or macro climate conditions for other than a very short period, Mt St Helens may have affected the atmosphere for a year and Krakatoa for several years but overall providing the model includes the effects of normal annual volcanic activity, and that would include all Level 4 eruptions because of their frequency I cannot see that its failure to take into account much less frequent larger eruptions will significantly affect its use as a forecasting tool. It will only be one item in a range of resources used by the IPPC inmaking their decisions