My niece Erin, who has studied in Irleand, is expecting her first baby. Her parents want the baby to call them the Irish equivalents of grandmother and grandfather. I would appreciate it if someone could send me the Irish terms, including pronunciation, and the commonly used Irish diminutives (e.g, for Grammy or grandpa.

grandmother is seanmháthair, grandfather is seanathair.

But: my children call my mother Nana and my father Daideo.
Mamó is another commonly used version for Granny.
I don't think children would call their grandparents seanathair/seanmhathair, that would be a bit too formal.

Aonghus,
Go raibh míle maith agat.
Steve Rojcewicz