When Cathal Armstrong was a child, his family bought all their fish on Fridays from "the ladies on the pier" in Dun Laoghaire. On Saturdays, if they were lucky, the Armstrongs would have lobster for lunch. In this salad, Cathal combines the sweet lobster with fresh fennel. His tip: Soak the sliced fennel in icy lemon water before serving to make the pieces extra-crisp.