"Irremissive" means something that cannot be remitted or overlooked, implying that it is essential or unavoidable. Synonyms for irremissive might include "inescapable," "inexorable," "unavoidable," or "inescutcheon." Other possibilities include "unforgivable," "indispensable," "inescapable," or "inviolable." Depending on the context and usage, different synonyms may be more appropriate. For example, if discussing a necessary task or obligation, "indispensable" might be a good synonym, while if describing something that cannot be forgiven, "unforgivable" would be a better choice. Ultimately, choosing the right synonym for "irremissive" will depend on the specific context of its use.