I think the only difference could be the cost, during the school summer break the ferry prices and B&B's put up the cost .
The weather just roll the dice and take a chance , i have had some great warm times in September and others bloody freezing. Usually not to bad .
My suggestion would be to follow the small roads (most of them were the main roads before joining the EU )
Come of the ferry , drive out, be doubtfull if you get stopped on a bike by customs and head for Wexford go left to Wellington bridge and Passage east (small ferry over the river runs every 20mins and a couple of euros) on the R733 Follow that to Waterford (cyrstal factory) . Follow the N25 to Cork then the N71 to follow the coast via Skibbereen, drink Guiness
and eat Lobster and listen to the tall tales .... job done
On some roads on the left you will notice a solid white line( like a hard shoulder) it is custom for slower traffic to move over into this lane when safe to allow faster vehicles to go by , if they move over just give em a wave and if your holding up someone move over !
Another interesting point is when you arrive on the ferry if you look to the left just before the harbour that beach was used to film some scenes from Saving Private Ryan.
Hope this helps