Nonreplaceable means that something cannot be replaced, but it may not necessarily be valuable or important. Irreplaceable, on the other hand, means that something is not only unable to be replaced, but it is also incredibly valuable or important.

Irreplaceable means something cannot be replaced, and is often used to describe something of great value or importance. For example, a family heirloom may be irreplaceable. On the other hand, unreplaceable implies that something should not be replaced, even if it could be.