Which direction to travel out of Dublin?

Makes no difference which way you drive.

Have driven from Dublin to Glendalough, to Kilkenny to Cork on to Kinsale to Dingle to Doolin to Galway and back to Dublin.

Another drive from Dublin driving into NI from Belfast, to Derry, Donegal to Sligo, Clifden, Galway back to Dublin.

Last trip drove Belfast to Dingle in 4.5 hours.

Whichever route you choose...slow down and appreciate Eire’s beauty.

Makes no difference, though what we did (which worked out nicely) was to skip the first drive, and take a bis to Galway (we took the bus on arrival at the airport, since we knew the rest of that day would be jetlagged and of limited use). We picked up our car in galway, drove (a lot) counter-clockwise. When we were wrapping things up, we dropped our car in Cork and caught a train to Dublin. Worked great for us.

Per the others, the direction of travel doesn't really matter.
The only difference I can see is whether, as per David, you'd like to pick your car up in Galway at the beginning of your trip, which many here recommend as a strategy to avoid hitting the road immediately upon arrival in Dublin... jetlagged and sleep deprived from the flight over. That obviously won't be a consideration for you since you'll already have a few days to acclimate before setting out.
If you choose to start clockwise from Dublin, ie via Glendalough & Kilkenny, then you'll have the option of finishing up your driving adventure in Galway, returning your car there and then train or bus back for your return flight.

Thanks everyone!