Road trip from Dublin at Christmas, 8 nights

Have you considered taking the bus to Galway upon arrival in Dublin? The bus stops right outside the terminal. You'll get a little tour of Dublin while the bus makes it way out to Galway. And you can nap or enjoy some scenery. Spend your first few nights in Galway then rent a car and continue on your trip. If you really want to rent a car upon arrival you could stay some place like Trim which has a castle and monastic ruin.

There's really no way to fit everyone plus their luggage into a single vehicle, so you're probably looking at 2 cars to accommodate everyone.
Most here agree that it's unwise to start driving immediately upon arrival, ie after a long overnight flight when you'll likely be sleep deprived, jet lagged, and generally not fit to be behind the wheel. Spending a night in Dublin to rest up a bit before tackling your driving adventure would be the more prudent choice.
An alternative would be to reverse the order of your trip and start in Killarney, taking advantage of one of the two daily flights from Dublin direct to the Kerry airport about 20 minutes from the town. Could then spend a few days there and/or Dingle before heading up to Galway, either self-driving or booking day tours from Killarney. There are several rental car outlets at the Kerry airport. Could do a one-way rental from there back to Dublin, or maybe dump the car in Galway and train back.
You'll be dealing with very short days and likely some marginal weather in December. Here's a link to last December's Dublin Weather for info.

I did consider bus to Galway but I’ve not found one running air coach seems to have cancelled route. If there’s another option I’m all ears….is it easy to grab car in Galway?

Otherwise…we would miss morning flight to Kerry and there’s no appetite to spend day waiting for a 4pm Ryan air flight on our first day. Not really an option. Also, will be renting Renault van - 7 seats and room for 4 suitcases. As a family of six we are always traveling right and have packing to a science. Of course, wr will be unpopular on the roads and pulling over constantly - two cars would be a nightmare.

To clarify —- if I were to reverse, the question is how much of a drag would it be to spend 2 nights in Dublin and then pick up car (at airport? Dragging bags from city center?) and head west. We’d have to spend last night in Dublin before flight (I guess near airport if available)

Anyhoo — I’ve booked accommodations in a panic as there’s SO SO little availability. All can be cancelled. As of now I have

Athlone 1 night ( no clue what there is to do in athlone, but it had hotels unlike Wicklow and isn’t as far as Killarney). I’m not wild about this but also not sure what would work better

Cliff of moher - 2 nights

Killarney - 2 nights (plan is dingle and ring road)

Dublin 3 nights

Ps. Trim is actually a great suggestion. I’ve been. Thank you. Does it take us annoyingly out of our way though? limited accommodations but could make it work and maybe better firsts day than athlone?

no clue what there is to do in athlone

Athlone is an attractive town on the River Shannon. Athlone Castle is open in December and gets consistently good reviews on Trip Advisor. Near it is the impressive Church of St Perter and St Paul which would be worth a quick look inside. Have a look at the virtual tour here.

Only a few miles away is the early Christian site of Clonmacnoise, again open in December, with its ruined chapels, round towers and carved crosses. This is definitely worth visiting and is a lovely setting on the River Shannon.

Trim only puts on a few extra miles so would work as your overnight stop. You could still do Clonmacnoise on your way to Galway .

When we did a road trip in Ireland, we picked up our car in Dublin City Center. We stayed at the Holiday Inn Express City Center (O'Connell Street Upper) and picked up our car at the Sixt Car Rental office a block away. We returned it to the airport the evening before we flew home (stayed at the Carlton Airport Dublin Hotel). There was no extra charge for picking the car up in town and dropping it at the airport.