The clincher came when an Ards attack broke down inside the 'Stones 22 with McNaughton collecting the loose ball and sprinting away for the vital try. After that Stewart Paul landed a second penalty to leave his side within a score of the lead but there was to be no fairytale ending for the northern club as Greystones battened down to hatches and held on for victory.
Ards opened with a try from full-back Paul after just six minutes and might have improved on that shortly after when they had a three-on-two but were denied by fine cover defence from wing Stewart O'Connell.
Out-half Neil McCarthy opened his side's account with a 23rd minute penalty and then edged 'Stones ahead wih another successful effort just before the break.
Ards looked the more threatening in the second half but were unable to finish and they only had a 60th minute Paul penalty to show for their dominance.
The game turned in the 70th minute when Scanlan finished off a sweeping move and then McNaughton made the crucial intercept to seal the outcome.
Ards:S Paul; N Beggs, S Devoy, N Stewart, R McClements; S Bell, N Hollinger (C Allen 79); P Eccles (G Garrett 65), R Fisher (D Herron 79), J Edwards, G Presho (J McCavery ht), D Tweed, M Creighton (K Heaslip 65), M O'Shaughnessy, J McDowell (K McIlwaine).
Referee: D Keane (IRFU)