My best bet is perhaps "summer" and "winter" got swapped? That's a simple single word mistake and -10 C would seem to be about the winter average in Ireland during the last ice age from other sources I can find, regular dips to -20 C at night or particularly cold days would seem quite reasonable. Based on any other sources I can find, thea peak of summer should have been slightly warmer than freezing (which is not incompatible with year round glacial coverage, it takes a lot of energy to melt such enormous volumes of ice).

I've sent Dr Nash a short email asking if that section of the article was accurate in hopes he's willing to clarify one way or the other.