Also factor in that Dublin has flights to all 5 London airports (plus Southampton and Southend). Seeing that VFR is one of the largest sources of passengers for London flights and that those people being visited could live all over the London area, flying from Dublin can make sense.
In my own case, my sister lives in South-West London and I would only use Heathrow (20-30 mins) or Gatwick (45 mins) to visit her. Luton or Stansted would take over 2 hours to get to her house after landing.
Were I Waterford based, what I'd gain on not traveling to Dublin, I'd lose at the other end.