I can answer my own question now.
Apparently the A320s that were on the ground overnight were "contaminated with volcanic dust". So, the Heathrow and Amsterdam flights were waiting for replacement aircraft to be flown down from Dublin. Meanwhile, the Malaga and Faro were cancelled as a result.
All that meant that when those two did arrive down from Dublin and the Gatwick flight had come in that there were seven Aer Lingus A320s on the apron. If only that was a more regular sight!
What I don't get is why whatever dust fell knocked out the Aer Lingus A320s, but Ryanair were able to use their 737 to fly to Dublin. Any ideas?