Malahide Castle & Gardens is a real time commitment — fit it in only if it's a priority.
Allow Half day (3–4 hours).
A storybook castle with a guided interior, a lovely walled garden and butterfly house, and grounds that make for an easy family half-day reachable by DART. The honest catch: the interior tour is short and the castle is fairly modest inside, so the real draw is the grounds — less compelling on a wet day.