"I'm not sure what "righteousness" you're talking about"

Mostly the idea one would host the guy but only if he didn't have a car. Which is reasonable in terms of the Warmshowers criteria, or possibly a lack of parking... Possibly.

Efficiency wise, I'm sure you are right at some level. I lived in Dublin for a year, so I wouldn't want to spend at least two of my days traveling the congested routes of that town. It's a great place so a person who has never been might not want to miss it. On the other hand, take a rock climbing example. You could use local transit and do Dalkey Quarry, or rent a car and do Donegal to the North, and probably hit virtually every other significant crag in the place during your trip. There is a reason we have cars. It really comes down to whether cycling is the ideal or the objective. If I was on a fly fishing trip to Ireland, I wouldn't limit myself to areas within transit o Dublin, though the Liffey has some schooled trout

Let's say you end up doing a cycling loop in any case. I'm not sure the logic that every loop best starts at the airport is valid. And any other place you choose that is more than 20 miles from the airport, will be faster by car than transit. Will it cost you more money to use a car, sure, but if that was such a big deal you won't be jetting it. Still been a while since I rented a car in Ireland...