TheFork users who ate in L'Isle-Adam have said really good things about L'Affiche, Les Vignes Rouges and La Route du Kashmir. Maybe you should give them a try!
Some of the more popular Christmas and New Year restaurants having a terrace in L'Isle-Adam according to TheFork users include:
L'Affiche, with a 8.7 rating
Les Vignes Rouges, with a 8.9 rating
La Route du Kashmir, with a 9.2 rating