I have been doing this in stages since 2021 from Co Fermanagh... switching to the Voie Littoral near Saintes to Biarritz then onto Via Bayona to Burgos. A few others from Ireland have done similar.
Just FYI thru Ireland I started on Ulster Way to Blacklion, then the Cavan and Leitrim Ways to Leitrim Village before a road walk to Cloondara and the Royal Canal Way which took me right into Dublin mainly off road.
There are options quite a few campsites, and apart from a few section in Landes and near "resort" areas wild camping is possible. But From Mont St Michel there is a well-developed chain of accomodation to Nantes for pilgrims, and thru Vendee there are a list of pilgrim hosts (most are okay with little French).
The local associations are great for info and support, and the markings from Mont St Michel are pretty good on the Voie des Capitales (I found in Loire region they were a bit less frequent, and downloading tracks is useful). There is a marked pilgrim route from Cherbourg to Mont St Michel, which apart from in some towns is fairly well marked. (Nb use the version of their website in French as English version has less information including accomodation information)
Site de l’association les Chemins du Mont Saint Michel
www.lescheminsdumontsaintmichel.com
I definitely recommend crossing the Bay fro. Genets to Mont St Michel and book to stay in the pilgrim accommodation on the Mont.
As for the relevant local associations- start with the Bretagne (Brittany)
compostelle-bretagne.fr
in Vendee
www.vendeecompostelle.org
In loire Atlantique
Bienvenue sur le site de l’association Les haltes pèlerines en Loire-Atlantique & Via Ligeria. Vous trouverez sur ces pages, des informations pratiques sur les chemins pèlerins traversant notre département, vers le Mont-Saint-Michel depuis Clisson, vers Rome depuis Nantes mais aussi vers...
haltespelerines44.fr
Hope that is some help - I also tried to keep a daily "live from camino" on the forum if you are interested. More than happy to try and answer any questions.