Thornton, whose marathon best is 2 hrs 17 mins, finished in 3 hrs 49 mins 29 secs. The race, with 46 runners from 20 nations starting, took place around 20 miles south of the magnetic North Pole over a 4.6km lap hacked out of the snow.
Now 33, Thornton spent some time in the Dublin and the UK before returning to Galway to work as a primary school teacher. He is a former national 10,000m champion and took a year off work last year in an effort to qualify for the marathon at the London Olympics. His wife Elaine Barrett also travelled to the North Pole, completing the half marathon.
* Watch a video of the race at www.guardian.co.uk