As well as the Collins,
slieve is in the American Heritage dictionary. It doesn't seem to be in any of the Oxford dictionaries, or in Wiktionary.
Usage examples are pretty easy to find in publications from Ireland and, to a lesser extent, the UK. However, about 99.9% of these examples are in place names, and hence with a capital S - e.g. Slieve Bloom, Slieve Russell. An example using the word as a common noun comes from an
online guide to Ireland:
Mountains in Ireland tend to be stacked around the edge of the country. Most bear the Irish name for mountain, which is slieve. Hill-walkers can enjoy long-slieve and short-slieve routes, as well as parts which are entirely slieveless.
I'm not thinking of adding
slieveless, which is obviously just part of the author's joke, but I'll add
slieve as a rare word.