An irreversible crimp is basically a permanent mechanical connection that's made using a crimping tool designed to compress a metal sleeve or lug around the wires. Once it's crimped, it can't be undone without cutting it off — hence "irreversible."
Inspectors ask for it because it's more reliable for grounding than something like a wire nut, which can loosen over time. For solar systems, they're usually looking for copper compression lugs or grounding crimp sleeves that meet UL 486A-486B standards. You'll need a proper crimping tool — not just any pliers — to make sure the connection's solid. Burndy, Ilsco, or Panduit are common brands for this kind of stuff.
Wire nuts won't cut it for grounding in a solar setup — they're just not secure enough long-term.
