This is a service method that for a witness set W sets the flag W.cache.IsIrreducible to true.
The object declareIrreducible is a method function.
The source of this document is in /build/reproducible-path/macaulay2-1.25.06+ds/M2/Macaulay2/packages/NAGtypes/doc-NAGtypes.m2:666:0.