Sago
If your doing it as a grandchild of an Irish citizen you need to go on the foreign birth register first, this takes 12 months plus and costs a few hundred euro.
You may then apply for an Irish passport.
I did it simply as my father was born in Ireland, I got birth and marriage certificates, filled in the application and got my passport within 6 weeks, it was under 100 euro.
My SiL's father was born in Ireland and, when he applied for nationality he was told "you don't need to, you've always been Irish even if you didn't realise it". He got his passport in about the same time as you mention.
He, and DD, are currently registering DGC on the foreign birth register.