yeah...NO. It's the island's defenses, not derived from anything of Harry's or Molly's. Harry was very specific about that near the beginning of Skin Game (reviewing his options as he went down to meet Mab on the dock).
I suppose it *could* be a "simple keep-out dome" -- there were no details -- but Harry used the term "defenses" (implying an array of methods & tools) rather than saying "shield" or "ward" or the like.
If you set your defense and they can't break it, there's no rush to target multiple individuals at once. Area effects work.
If you can dome out to the shore, the magic users are out over water. That doesn't help them. If you can, have the equivalent of murder holes to attack out. The magical equivalent of mines released. Greek fire. If not for the question of suborning will - maybe some of the prisoners are aquatic. Fight for me, maybe get a parole. Claymores on the land - how would the fae react to the BBs? layered defenses. But when Harry faced the SC - he was right in front of them, and they were within these perimeter defenses. You let them in close, harder to defend. They get close, even some guy with only half of one hand can set a booby trap and almost get you.