Rental pickup locations outside Dublin?

Dublin Airport. A bus will get you there, and the highway skirts Dublin so you don’t have to pass through downtown. The extra charge for an airport car pickup is the trade off for dealing with city traffic involving another pick up location.

I agree. Take a bus to the airport. You can bypass the city as you leave.

FYI: Optional PMI Insurance is required in Ireland. Find out he PMI cost before you complete making your reservation. Irish PMI is not as inexpensive as in the other country where the coverage is required--Italy.

Be aware that driving in rural Ireland can be pretty tough. Roads are roughly paved, and crooked. Deep ditches and hedges often run close to the pavement. But it's the best way to see everything you want to see.

Without knowing your proposed itinerary or the duration of your visit it's difficult to speculate on what would be the best strategy for you.
In general I agree that collecting your vehicle at the airport will likely be the most convenient option. There will be a better chance of getting exactly the type of vehicle you prefer since they'll have the largest inventory in the country. Be aware that the rental car offices aren't at the airport however, but about a half mile away along Swords Road. If taking a taxi to pick up your vehicle on getaway day you'll want to ensure that your driver knows that you want to go to the rental car lot (it's huge) and not back to the terminal. Might also mention that others here prefer to use one of the rental car offices on South Circular Road - from which it's a straight shot out of town ... so no city traffic to worry about. Would be more convenient if you're heading south towards the Wicklow Mountains for example.
That said, many here suggest skipping Dublin upon arrival and saving your days there until the end of your trip rather than at the beginning - the alternative being to take the express bus that goes from the airport over to Galway and spend a night or two there before collecting your car for a one-way rental finishing in Dublin. That way you sort of steal a day at the beginning of your itinerary to deal with any jet lag or sleep loss issues before hitting the road well-rested and fit to drive. Could also catch the plane or ferry over to the Aran Islands one day and thus save another day of needing a rental at all - not a minor consideration with automatics going for over $100 per day at the moment.
We found that driving in rural Ireland was a delight, but if you're at all worried about conditions you can preview your daily routes by taking advantage of the Street View feature on Google Maps. It's always really helpful for me to have seen my planned route in advance - might help you too.