All applicants for the De Facto Partner Visa to Ireland have to apply for Immigration Preclearance, granting them permission to apply for a visa and enter Ireland. If your application for Irish Immigration Preclearance is successful, you will be issued a preclearance letter, giving you 6 months to enter Ireland. If you do not arrive in Ireland within these 6 months, you will need to apply for Preclearance once more.
If your application for Preclearance is unsuccessful, you will not be allowed into Ireland.