The Verdict: Clear Path
With a grandparent born on the island of Ireland, you can become an Irish citizen by entering the Foreign Births Register. Apply to the Department of Foreign Affairs; its published processing time is around twelve months.
Your parent was an Irish citizen from birth through their Irish-born parent, whether or not they ever claimed it and whether or not they are alive. Because you were born outside the island to a parent also born outside it, you must register, and your citizenship starts on the date of registration.
One check: if your grandparent formally renounced Irish citizenship by lodging a declaration of alienage before your parent was born, nothing passed down. Taking another nationality is not renunciation, so this is rare.
You need your own civil birth certificate, your parent's, your Irish-born grandparent's, and any marriage or change-of-name certificates linking the surnames. Originals are required, and a witness from an approved profession signs your form and photographs.
If you have or plan children: they inherit only if you are on the register before they are born. If a birth is expected, say so when you apply, as it can be treated as urgent.