The mystery athlete appeared to be so far ahead of the field that none of the other competitors even saw him ghost past them.
Henry O'Neill was not seen by any of the race stewards but he was clocked as the leader at 10km and again at the half-way point.
The progress of runners were tracked on websites by referring to the positions of their timing chips on the course and the system showed '32 H O'Neill' to be ahead of the pack.
Messages online began to ask: "Who is this Henry O'Neill leading the race at 10k?"
But Mr O'Neill might have been the most intrigued of all because he was at home in Co Tyrone during the race.
He is an elite marathon runner who had registered for the race but he had to pull out before the race date.
But the organisers had placed a bag containing his unused race number and timing chip onto the press bus which drove around the course just ahead of the runners.
The chip continued 'bleeping' its position on the tracking systems.
But then the chip seemed to vanish from the race after the bus ended its journey just before the end.
No one on the bus had realised the confusion being suffered by race web trackers. The race was won by Sean Hehir.
It turns out Henry, who works as a vet and lives in Omagh, is a top runner who recently won the Cookstown Half Marathon.
And his 'ghost' just nearly won the Dublin City Marathon, too.