Ross Adair starred for the hosts, hitting 21 runs with the bat before taking three wickets with the ball to set up the win.
Having been inserted by Andrew White, the CSNI batsmen found it difficult to establish a partnership with Marc Ellison (5), James Kennedy (1) and Graeme McCarter (2) all falling in the powerplay with the scoreboard reading 31 for three after six overs.
Instonians’ bowlers bowled accurately, building pressure and made it difficult for the home side to get the ball away and find the boundary.
Corin Goodall top-scored with 25 and Ben Adair (10) was the only other man to reach double figures as CSNI closed their 20 overs on 88 for eight. Shane Getkate was the most successful bowler for Instonians, finishing with figures of three for 21 off his four over allocation. There were two wickets for Matthew Peake and a wicket apiece for White, Robert McKinley and James Magee.
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Chasing a seemingly low target of 89 to win, Instonians slumped to 21 for five with Getkate (10) being the only man in the top five to reach double digits.
White and Ben Rose absorbed the pressure and shared a sixth wicket stand of 46 before the former found Ellison off the bowling of Ben Adair, departing for 23. The visitors were in a strong position, needed 18 off 13 balls with four wickets in-hand.
McKinley and James Metcalfe fell in successive balls soon after and their side still required a further 10 runs off the remaining five deliveries.
Ross Adair held his nerve in the final over, only conceding four runs before having Rose stumped for 33 off the last ball of the match.
CSNI host Lisburn at Stormont next Saturday, meanwhile Instonians travel to Comber to take on North Down.