'Early' has long since gone, for Dublin you should always buy as soon as possible and since you want to travel in 4 weeks time, don't hesitate, there are far fewer flights on that route than the ones from Heathrow or Stanstead, so fares are often not as cheap anyway

is it best to get flight tickets early or last minute?

i'm looking for the cheapest deal for a flight from gatwick to dublin (return) on 30th nov this year.