The Letterkenny side had gone in at half-time 3-2 up thanks to a couple of spectacular goals.
But the Deele lads came back with a couple of crackers of their own to win 4-3.
Donegal Saturday League Results
Downtown Cup Group A
Glencar Celtic 3 Deele Harps 4
Ramelton 3 Glenea Utd 1
Downtown Cup Group B
Castlefin Celtic 0 Milford Utd 1
Fintown Harps 4 Dunfanaghy 1
Downtown Cup Group D
Cranford u 1 Gweedore Celtic 1
Gweedore Utd 2 Downtown Utd 4