From Jersey in the Channel Islands it is possible to travel as a foot passenger to Granville in France but only in the summer months withs Manche Iles Express. Then take a train to Pontorson which has a bus connection to Mont Saint Michel (or a 10km walk along a canal).
The other option is ferry from Jersey to St Malo with Condor. It runs a few times a week. From St Malo take a train but you have to change at Dol de Bretagne to reach Pontorson (with a bus to mont St michel). There is also a hiking route from St Malo to Mont St Michel
After Mont S Michel - From Pontorson there are two possible routes to Irun - first is to take a train to Paris, then to Hendaye and then switch at Hendaye or walk over bridge into Irun - this is fairly fast and straightforward. The other involves a longer series of different trains - to Rennes, then Nantes, then Bordeaux then Hendaye (all of which are lovely cities to visit) - but this will take alot longer and probably only if you want to sightsee along the way.
The other option is much longer - there is a camino route marked from Mont Saint Michel (2 in fact!) - I did the Voie des Capitales from there then connected to the Voie des Anglais/Littoral to Bayonne and onwards to Irun. But that is at least a month's journey
