I think it's both - some people spell it with one word, some with two. I don't know that one's correct and the other not.

It's a cupboard in which is contained an immersion boiler, nowadays usually insulated. Above this sit three or four slatted shelves, set widely apart, on which you can pile folded (hopefully) clothes and bedlinen and towels and so on. Also known as an airing cupboard. A fixture in most houses in Ireland.