Hi NewGram,
The formal Irish words for grandmother are seanmháthair, máthair mhór or máthair chríonna.
But Irish speaking children would usually use mamó as an informal and affectionate way of referring to their grandmothers.
Mamó would be along the same lines as granny in English.
If directly addressing their grandmother, they'd say a mhamó.
The reason for this is when we address someone in Irish, words change slightly. For example, a boy named Seán would be addressed as a Sheáin, or a girl named Bríd would be addressed as a Bhríd. So when talking to your granny, you'd use a Mhamó. When talking about her, you'd use Mamó.
But maybe I'm overly complicating things. As your future grandchild won't be speaking to you in Irish, he or she could probably just address you as mamó and keep it simple... :)
You can hear the pronounciation for mamó here... http://www.teanglann.ie/en/fuaim/mam%C3%B3
(For the pronounciation of a mhamó, the a sound is a bit like uh and the mh sounds like a w. So you have something like uh wahm-OH).
English speaking Irish kids usually use Nana when referring to their grandmother.
These terms of affection, Mamó and Nana, are not just for kids though. Many Irish adults continue to refer to their grandparents as Mamó or Nana.
Hope this helps.