If you have renounced your Irish citizenship, you can only reclaim it if you were born on the island of Ireland, by making a declaration of citizenship.
To renounce your Irish citizenship, you must have completed a declaration of alienage and submitted it to Immigration Service Delivery.
If your circumstances change and you want to reclaim your Irish citizenship, you can make a declaration of citizenship.
You need to:
Complete a declaration of citizenship Form
Get your declaration witnessed (there’s a list of acceptable witnesses on your form)
Submit your form to Immigration Service