Rennes is a very well connected city.

Arriving by train

Rennes train station has direct connections to Paris, Bruxelles, Lille, Nantes.

Once in Paris, the trip from Gare Montparnasse to Rennes takes 1h30mn.

You can buy train tickets from the SNCF website

Arriving by plane

There are direct flights from Paris, Dublin, Amsterdam, Francfort, London,...

Travel tip: If you land in Roissy, there is a direct train leaving Roissy CDG2. The trip from Roissy to Rennes takes 2h30.
Compared to a transfer to another flight, or to a train leaving gare Montparnasse (with 1h of public transports), this option is wortn considering.