Just had our second kid arrive, so tight on time,but wanted to give a few key points as I'm from Dublin.

Phoenix Park in city centre is good option for a spin, lot of cyclists train there,its a few miles of a loop,basic enough but does the job. You can search for the route online,stay off the main central road as much as you can, people and cars everywhere.

Do yourself a favour and make time for a 3hour spin:that gives you time to reach the Sally Gap, good climbs and stunning scenery. Lookup Lough Te near Sally Gap, Vikings show and King Arthur movie were filmed there,stunnning view of it from the road above.

Rentals: this page has some links I'm not familiar with but worth a look http://dublinbikeblog.com/...-a-bike-in-dublin-2/

Pretty sure this is the crowd I rented a bike for a work colleague from a couple of years ago (I also work for a US MNC based in Dublin): http://phoenixparkbikes.com. their site doesnt show road bikes but pretty sure they do some basic road bikes so give them a call.

Post here or PM if there's anything else I can help with. Cyclist magazine did an article on the Sally Gap route I mentioned last year: they were surprised that such a stunning route could be reached so easily from the city centre,so I recommend trying to fit it in. Prepare for all weather though,even if Dublin is sunny, the hills of Wicklow can get cold and wet quickly.