1. Place potatoes, 2 cloves garlic, and salt in large pot, and cover with water. Bring to a boil, reduce heat to medium, and cook 25 to 30 minutes, or until potatoes are very tender. Reserve 1 cup cooking water, then drain. Discard garlic, and return potatoes to pot.
2. Press remaining clove garlic through garlic press, and warm with oil in small saucepan over medium heat until garlic begins to sizzle, then add dulse, and cook 1 minute more. Remove from heat, whisk in parsley, lemon juice, and 2/3 cup reserved cooking water; pour mixture over potatoes. Smash potatoes into rough mash, adding more cooking water to achieve desired consistency. Season with salt and pepper, if desired, and garnish with extra dulse and parsley.