We have done this a few times and I really recommend Brittany.
THe scenery and landscape is lovely, and the food and holiday facilities are great.
In late June, going a bit further south is a good idea.The weather in Northern Brittany is mild with a risk fo rain at that time of year- like Ireland only warmer, but not hot.
Southern Brittany is similar but a bit warmer and drier.
We stayed in St Pointe de Gilles Holiday park in Benodet, and it was excellent.
There is no need to book with Keycamp or Eurocamp,you can book directly with the site. Benodet is about 150km from Roscoff.
http://www.escale-stgilles.fr/
Keycamp and Eurocamp are more expensive Unless you really want the kids camps, I wouldn't bother with them.
Benodet is a nice town and there is lots to do nearby.
Brittany Ferries go from Cork to Roscoff but the ferry only goes each Saturday, so you would have to go for 1 or 2 weeks. Irish Ferries from Rosslare have ferries on different days and are a bit cheaper.
Some campsites insist that you can only check in or out on Wednesdays or Saturdays/Sundays but this may not be an issue in June or early July. Hig season in France starts on Bastille day (July 16th)