Cost of Cape Town flight from Dublin vs from Heathrow

Hi there,
Recently I saw a post on TripAdvisor where someone had said they purchased a Virgin Cape Town flight far cheaper from Dublin than from Heathrow, even though the Dublin flight goes via Heathrow.
I'm planning on taking my family to South Africa this Christmas so thought I would check this out. It is true. Dublin - Cape Town return via Heathrow leaving December 21st costs £1300. Heathrow - Cape Town (same flight) is £1800 return. Can anyone explain this? It seems crazy?
When multiplying this saving by 4, it seems worth it but a huge inconvenience as we have 2 young children!
Any insight into this phenomena would be much appreciated!
Recently I saw a post on TripAdvisor where someone had said they purchased a Virgin Cape Town flight far cheaper from Dublin than from Heathrow, even though the Dublin flight goes via Heathrow.
I'm planning on taking my family to South Africa this Christmas so thought I would check this out. It is true. Dublin - Cape Town return via Heathrow leaving December 21st costs £1300. Heathrow - Cape Town (same flight) is £1800 return. Can anyone explain this? It seems crazy?
When multiplying this saving by 4, it seems worth it but a huge inconvenience as we have 2 young children!
Any insight into this phenomena would be much appreciated!