Both seem to be small islands in unknown territory, summer would suck winter would be way worse. How did they keep supplies and food? How was a small dislodged group able to hold off in these conditions?
At least for the 1st cases (Northmen in Island Noirmoutier), West Frankish Courtier Prudentius record one interesting details in the Annals of St. Bertin, a. 843:
'Northmen Pirates attacked Nantes, slew the bishop and many clergy and lay people of both sexes, and sacked the city (civitas). Then they attacked the western part of Aquitaine to devastate them too. Finally they landed on a certain Island, brought their household from the mainland (convecti a continenti domus) and decided to winter there in something like a permanent settlement (hiamare velut perpetuis sedibus statuerunt) (Nelson trans. 1991: 55f.).
This passage suggests that the Norse raiders could somehow get the building material for houses (domus<domibus), not hasty made dug-huts, possible with the palisade, in the adjoining mainland. It also implied the possible existence of not so 100% hostile interaction between alleged Norse raiders and local neighbors in the mainland.
The attack of Nantes, mentioned in the 1st sentence of the paragraph, took place in the midst of the feud for the two local margraves (marchiones) between Rainald and Lambert (Nelson trans. 1991: 55, note 1). While Rainald was loyal to King Charles the Bald to fight against the Vikings in Noirmoutier in 835, his rival, Lambert, was not. One 11th century account, Chronicle of Nantes, suggests a secret alliance between him and the Viking raiders.
I cannot assert for sure that Lambert is the traitor who traded with the Northmen in 843, but these fragmentary sources at least show that not all Frankish local magnates were in fact boasting solidarity against the Viking invaders: a few chronicler even suspected that Emperor Lothair I connived with the Northmen to attack against the kingdoms of his brothers. It is likely that some of them, as well as the developing Norse wintering quarters across Western Europe, such as Dublin, were not so unwilling to supply the Northmen with food and water.
Later, King Aethelred the Unready (d. 1013) of England also repeatedly accused Richard I of Normandy of offering shelters to the Danish fleets in the turn of the millennium (Cf. Neveux 2008: 93f.), but I wonder whether it's possible to find a kind of its prototype already in the middle 9th century Francia, with the divided loyalty and power struggles of the royals as well as local magnates.
References:
Nelson, Janet L. (trans.). The annals of St. Bertin. Manchester: Manchester UP, 1991.
Neveux, Francois. A Brief History of the Normans, trans. Howard Curtis. Running Pr.: London, 2008.