Outside of private transfers, what are options for a shuttle (or similar service) from a major hotel in Dublin to the Dublin airport?
Alternatively, is there regular bus service from downtown Dublin to the airport? Taxis and "uber" type services seem very expensive.
Yes, there is regular bus service from Dublin to the airport.
https://www.dublinairport.com/to-from-the-airport/by-bus/dublin-buses
The airport bus works fine - especially if you're a solo traveler.
We prefer to just get a taxi for the two of us with our luggage - for the door to door service and the convenience of not having to sort out bus schedules or locating the nearest bus stop. The extra cost for two or more people is really pretty minimal.
I used Aircoach (https://www.aircoach.ie/). They were excellent. I booked my Dublin to airport trip online.
The Airlink express bus to the airport is very efficient if you are staying anywhere near one of their stops.
It may have been an unusual occurrence, but a few years ago we were on a crowded airport bus going to the airport that just drove by people standing at the stops for that bus for the last 1/3 of its route. I get it was full and couldn’t add any more people, but that was little consolation to those left standing by the side of the road with their luggage.