The Verdict: Clear Path
You never lost your Irish citizenship: Ireland allows dual citizenship and citizenship does not lapse. Apply for an Irish passport; there is nothing to reacquire.
If you were Irish by birth or descent, naturalising elsewhere costs nothing on the Irish side, however long ago it happened, and an oath renouncing other allegiances in another country's ceremony has no effect in Irish law. The only voluntary way to lose Irish citizenship is formally lodging a declaration of alienage.
If your Irish citizenship came by naturalisation, it is still in place unless the Minister has revoked it, but two grounds for revocation apply to you: voluntarily taking another citizenship, and living outside Ireland for seven years without registering each year your intention to keep Irish citizenship. Revocation needs a formal notice and a process, so nothing has happened unless you were told. Apply for the passport, and be ready to answer questions about it.
If you have an old Irish passport, even a long-expired one, renew with it, since it is the strongest evidence you have. If you never held one or it is lost, apply as a first-time adult with the document your citizenship rests on: your civil birth certificate showing birth on the island, or your Foreign Birth Registration certificate or certificate of naturalisation.
A lost Foreign Birth Registration certificate can be replaced: the Department of Foreign Affairs holds the register, and your entry does not disappear.