Plantains are bananas for cooking or baking. They are used in wide parts of the world as one of the main sources of nutrients. Once peeled they can be treated like potatoes.
Where to get it in Dublin[]
Plantains are easy to find in Dublin. Most exotic appearing shops would have them. Any African shop should have them and most Indian shops and Asian shops. You might even get them in Fresh or bigger Dunnes Stores or Tesco might have it. Sometimes even the market people e.g. of Moore Street Market would have some.