According to the model of convection rolls presented here, volcanic anomalies develop due to the effect of convection rolls effect on the tectonic plates. The complex pattern of currents underneath Iceland, along with the tectonic drift, causes the exceptional activity level of volcanoes and geothermal areas.
According to relevant articles, the hotspot should be found underneath the western part of the glacier Vatnajökull. It can be read about here: https://en.wikipedia.org/wiki/Iceland_hotspot
Geologists did search for a hotspot of Iceland, and came to the conclusion it should be found under Vatnajökull. This is a very active area, as two main volcanic zones meet there (East Volcanic Zone and North Volcanic Zone). The different trends of NE-SW fractures and NS fractures meet there, and the large volcanic sites of Grímsvörn, Bárðarbunga and Kverkfjöll are found there. Magma, specially from Bárðarbunga and Grímsvötn, has been found to flow far into the nearby polygons, causing vast eruptions. The hotspot is here seen as an active part of the interplay between convection rolls division lines, local rifting of the polygons and the effect of large scale tectonic drift of the tectonic plates of N-America and Eurasia.