Conor Coad gave the Waterford side the lead after 12 minutes with a super volley from the edge of the area before Patrick Fitzgerald equalised — after De La Salle failed to clear their lines — on 17 minutes.
De La Salle regained the lead on 28 minutes when Aaron Galibert sent in a brilliant left-wing cross that found the alert Kevin Flynn and the striker gave the Cashel netminder no chance.
Conor Whittle secured victory with a great individual goal, rounding the Cashel keeper before slotting the ball to the net for a 3-1 interval lead.
Cashel found it very difficult to break down a superb De La Salle defence, led brilliantly by central defenders Patrick Fitzgerald and Adam Potter
The second half producing no further goals but a number of opportunities that could have seen the Waterford college extend their advantage; Coad and Chris Kervick were both denied by the woodwork.
DE LA SALLE: S Whitty; M McManus, A Potter, P Fitzgerald G O’Donovan; A Galibert, C Whittle, C Morris, C Kervick; C Coad, K Flynn.
Sub: E Kindlon for C Morris (injured)
CASHEL: G Slattery, D Thompson, M Fox, M McDermott, C O'Gorman (c), C Condon, M Shortt, U Bedicis, P Fitzgerald, D Reid, C Conneally.
Subs: J Harding, R Anglim, I Collins.