This has already been posted documenting exactly what you need....
https://www.dfa.ie/passports-citizenship/citizenship/born-abroad/registering-a-foreign-birth/Grandparent born in Ireland
Documents you must submit with your application:
Your documents: Original long form civil Birth Certificate (plus photocopy)
Original civil Marriage Certificate (plus photocopy) - Church certificates cannot be accepted
Certified/Notarised copy of the photograph page of current passport or state issued identity document (e.g. Driver's Licence)
If 18 years or over - two separate documents showing proof of residence e.g. phone bill, gas/electricity bill.
If under 18 years – A school report/doctor’s letter showing proof of residence of applicant and two separate proofs of residence for the applying Irish parent.
Your parent’s documents (through whom you’re claiming citizenship) Original long form civil Birth Certificate (plus photocopy)
Original civil Marriage Certificate (plus photocopy) - Church certificates cannot be accepted
Certified/Notarized copy of the photograph page of current passport or state issued identity document (e.g. Driver's Licence) or original Death Certificate if deceased (plus photocopy)
Your Irish-born grandparent’s details: Original long form civil Birth Certificate (plus photocopy)
Original civil Marriage Certificate (plus photocopy) - Church certificates cannot be accepted
Certified/Notarized copy of the photograph page of current passport or state issued identity document (e.g. Driver's Licence) or original Death Certificate if deceased (plus photocopy)
Translating your documents
Please submit certified translations of any of the documents (plus a photocopy of each document) listed above if the originals are in a language other than English or Irish.
Further documentation
We may ask you to submit more documents to support your application.
Checklist
When you submit and print your online application form, a checklist of the documents you need will also print out. You need to:
Make sure that you have paid online
Make sure you’ve included all the documents required plus one photocopy of each document on the checklist with your application
Remember…
If you haven’t included all the requested documents, your application may be returned to you.