It's kind of hit or miss, sometimes it comes back and other times it doesn't. Would consider it a tender perennial.

It is hardy down to zone 4 so NOT a tender perennial!! If it doesn't return after winter, it is far more likely due to something amiss with its planting conditions than lack of hardiness.

Keep in mind that it is not a true moss but a low growing, creeping perennial (related to pinks and carnations). It likes a rich, moisture retentive soil and a lot of sun but dislikes heat so siting in a partial shade location (or in afternoon shade) is ideal. It is not at all drought tolerant and dislikes drying out but also will not tolerate very wet soils either.

Ah, that's my bad then. I've just never had such luck with Irish & Scotch moss..