Belle-Isle-en-Terre

Location of Belle-Isle-en-Terre in Brittany

Belle-Isle-en-Terre means 'beautiful inland island.' This could easily be the whole story. The name contrasts nicely with Belle-Isle-en-Mer in Morbihan and there are plenty of precedents for inland islands in France. But it is not. Belle-Isle-en-Terre is derived from the Breton Benac'h and that makes a simple sounding name a lot more difficult to decipher.

Belle-Isle-en

Belle Isle is an older spelling of what would be belle île in modern French, 'beautiful island', the circumflex indicating, as usual, the missing 's'. 
Like other places in France 'island' and 'inland' do not have to be mutually contradictory. Île de France (the District of Paris), Europe and France's largest regional economy,is an island simply because it is surrounded by water - in this case the Oise, Marne and Seine rivers. Lillemer near Dol de Bretagne includes île in its name (l'île muer) 'large island', despite being a good hike from the sea. However, Belle-Isle-en-Mer, near Quiberon and Carnac, off the south coast of Brittany is most definitely an island as we know it.

Belle-Isle-en-Mer

But the earliest records show that the origin of Belle-Isle-en-Terre's name are somewhere else. The current Breton name is Benac'h and this is much closer. Benisse was recorded in 1235 and then Benigla, 1330 and Bellisle was first recorded in 1477.

Ben can be derived from Old Cornish/Old Breton: ben ‘foot’ or ‘bottom end’ and it was originally contrasted with Old Breton: pen [also Welsh/Cornish: pen], which can be translated as ‘top’, ‘head’ or ‘end’.  
Nac’h is from Old Breton: cnoch [Middle Breton: knec’h, Welsh: cnwc, cnwch], ‘hillock’ . If Benac’h is ‘foot of the hill’ this contrasts nicely with Pen-an-Nec'h [compare this to Pencnwc in Wales], a lieu-dit at the ‘top of the hill’ near Belle-Isle-en-Terre . 

I say 'contrasted', but I'm sure it would be difficult to hear a real contrast between benac'h, 'top of the hill' and penac'h, 'bottom of the hill'. Wouldn't they get confused?

Maybe this was the Duke of York's problem.

Belle-Isle, then, becomes a simple corruption of the Breton name [Benisse→Benigla→ Belille→Bellisle]. And, to prove the point, the explanatory suffix en Terre was only added in the 18th century and then the real Belle Isle had to add its en Mer as well to avoid any confusion.