I suspect they are just hedging their bets on the winter rotations. Its a lot easier to increase than decrease. I'd be most surprised if this route was anything other than a big success.
Maybe there is an expectation that the traffic will be more biased towards leisure travel. 4 days a week in the winter is a slight disincentive to business travelers.
I've remarked that American airlines operating out of Dublin tend to operate a larger % of seats Business class, than do IE.
These flights can draw on traffic that would otherwise connect via Heathrow/Gatwick, drive to Dublin, or use florida/Canada charters. When you consider that connecting via Heathrow or Gatwick incurs almost £40 in taxes/charges alone, there is a large incentive to fly direct. I suppose that means few bargain fares
It's only a 172 seat 757, there will be many months with excess demand.