The entry was capped at 400 and race director, Brendan McDaid, has confirmed that no more entries will be taken.
"The organising committee took the decision this year to limit the entry to 400. We announced at the weekend that there were only 50 places left, and the last of those 50 was registered on Tuesday morning," he said.
"While the 5Ks and 10Ks locally have been attracting fields of 80 and 90, we have been pleasantly surprised at the demand for places in this year's Donegal Half Marathon. When we opened up the online entries in May, we were expecting an entry of around 300. But the high demand for places in this year's event shows just how popular it is in the racing calendar," he added.
"With the Dublin Marathon cancelled again for a second year, we're happy to be in a position to be able to hold an actual this year, as last year we could only go with a virtual race because of the Covid-19 restrictions.
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"This year's race will be held in line with all the Covid-19 guidelines and the start will see the participants going off in two waves of 200."
The main sponsor to the 2021 Donegal Half Marathon is the Kernan's Retail Group. The event is also supported by Donegal County Council, Donegal Sports Partnership, Optum, Fáilte Ireland and the Donegal Volunteer Centre.