What date did the Fai Cup with Derry City F.C. as runners-up take place?
CREATE TABLE "cup_finals_at_dalymount" (
  "date" text,
  "competition" text,
  "winners" text,
  "score" text,
  "runners_up" text
);
SELECT "date" FROM "cup_finals_at_dalymount" WHERE "competition"='fai cup' AND "runners_up"='derry city f.c.';