Car rental and what to pack

I can't answer your first question but for the second, I would suggest visiting the Packing forum. There are tons of posts there on packing light, and what to pack for special occasions. Believe me, it can be done.

Uber and Lyft are regulated and metered like taxis in Ireland and are actually more expensive than taxis because of the add on fees they charge. So while your Uber app will work to get a ride it will cost you more. Because public transit is fewer and more far between in Dingle, I would rent a car. Re dressing, Dingle is more casual than other places.

I've done a lot of planning for my upcoming trip, and I think I'd advise renting a car at KIR. You can get to Dingle via bus/taxi but there's a lot to do while you're in Dingle that will require day tours unless you have a car, including Blaskett Island, Conor Waterfall, Riasc Monastery, Eask Tower, Beehives, Dunbeg Fort, Gallerus Oratory, Famine Cottages, and others. You can bike them but it wouldn't be for the feint hearted. There is a Blaskett boat from Dingle Harbor, but I think the ones from Dunquin or Ventry are less boat and more Blaskett. A casual drive from KIR could include lunch in Killarney as well. I think you can probably spend an endless time in Dingle town if you like Guinness and music, but many folks think Dingle isn't "worth" more than a few days (not my opinion, since I'm in the former category).

You'll need to rent the car at the Kerry airport upon arrival, and you'll need it to explore Slea Head Drive and the rest of the peninsula once you're there.
From Kerry airport it's about an hour and 15 minute drive to Dingle town along the R561 - a two lane rural road. You'll want to be especially vigilant and make good use of your navigator to ensure a safe drive. Can preview the road and driving conditions by using the street view feature on Google Maps.
At any time of year layers are the way to go in Ireland, with the top one always being a goretex shell in case of wet weather. Per your other post the only way you'll make the morning flight from Dublin to Kerry is if you carry on and can avoid having to exit and re-enter the secure area.

We spent 3 nights in Dingle 4 years ago, ate at several nice restaurants. Jeans and shirt fit in fine with the crowds. Obviously you want something else for biking. Alternative for me was travel pants and same shirts. Easy peasy.

Thanks, everyone for all your helpful advice. As always, you guys are the best resource.