If we have a full day to do this drive, what are the main spots to stop, see, etc along the way?

ttt

We really enjoyed the Waterford Crystal factory, and also Blarney Castle, not far from Kinsale. Actually kissing the Blarney Stone is a bit cheesy, but it's a lovely site and surroundings.

I suppose you know that The Rock of Cashel is beside the Dublin-Cork road, and Cahir Castle is very close to it.

and if we leave before noon, can we see most of these before arriving in Kinsale that evening?

if you are staying in kinsale, do drive out to mizenhead/crookhaven.
so far, it's my favorite part of ireland.

Waterford is a bit of a detour -- too much, in my opinion.

What you can do, as you get near Cork, is head off left and follow signs for Cobh. If you have started early enough in the day, you get in some of Fota House, Fota Wildlife Park, and Cobh Heritage Centre, as well as seeing Cobh itself, which is an odd mixture of charm and decrepitude.

Rather than go back towards Cork to get to Kinsale, you can take a ferry across Cork Harbour, and take a quick trip down to Crosshaven, which is pretty, and then proceed to Kinsale.