Luggage and bags:
Suitcase/ ruck sac etc for your main luggage- a day-sac for sightseeing to carry your camera/ guide book/ binoculars etc
Clothing/Shoes/Weather Gear: Comfy shoes/boots for walking around.
Wear layers, the weather here is very changeable! Pack a waterproof - a good chance of at least a few spots of rain! I was here in March, and it was breezy but dry, then in July it was torrential rain-I was soaked to the skin, even through my waterproof jacket.
Stout walking boots for walking the coastal path or venturing onto the rocks.
Fleece or warm jumper.
Toiletries and Medical Supplies: Carry all prescription drugs etc, although there is a pharmacy here. (Next to the Spar Supermarket)
Photo Equipment: As much as you can carry!-photographers paradise! Waterproof case-
Miscellaneous: Binoculars for the views
Bubble wrap-for protecting jars of local chutneys etc.
lip salve/ moisturiser
Cool box/freezer blocks if self catering-for the freshest fish!
A raincoat and an umbrella are essential. You are in Ireland, of course! The day I was in Howth it was sunny, rainy and windy. Nice, huh?
Photo Equipment: Be sure to have something to protect your camera in the event of rain. Even if you have an umbrella, sometimes the rain goes sideways so you'll want to protect the camera.
Bring some wind gear because the wind blows hard at Howth Head.
Photo Equipment: You'll be able to get some great photos at Howth and Malahide. They say that "on a clear day" you can see back to Dublin, but the day I was there, it wasn't especially clear and we could see Dublin just fine.
Peter tells me that on a really clear day, you can see Wales from here. Now that would be something.