Can you pick up a rental car in Dun Laoghaire? If not where is the closest place?
I believe Enterprise still has an office on Pottery Road.
I would suggest Dublin Airport for rental car pickup & drop off. Most rental car carriers have shuttle services that will pick you up and take you to Dublin aerport.
I had a really BAD expierene in 1995 in Dubs; when I thought it would be 'eaiser' to pick up my rental(Avis) in Dublin instead of at the Dubs aerport.
If you don't ask if they have shuttle service you will never know.