The Verdict: Need More Info
Whether your Foreign Births Register entry was deleted or is being questioned, your entitlement may still be intact. Ask the Department of Foreign Affairs exactly what it doubts, or why it deleted the entry, and what evidence would answer it.
These cases usually turn on documents: a certificate that did not show what it was thought to, a miscounted generation, names that did not link up. The question is whether you were entitled: a grandparent born on the island, or a parent who was already an Irish citizen when you were born.
If the entry is only being questioned, answer with the documents before any decision is made. If it was deleted and you were entitled, the way forward is a fresh, properly evidenced application. If an application was refused, a letter of appeal is due within six weeks of the refusal.
Rebuild the chain: birth certificates for each generation down from the Irish-born ancestor, and marriage or change-of-name certificates for every change of surname.