Earlier, Sterling and his fellow opener Andy Balbirnie (45) scored 101 runs for the first wicket before their partnership was broken by Lizaad Williams in the 23rd over.
Curtis Campher increased the run rate after coming in at No 3 but was bowled by Ottniel Baartman despite looking in good form.
Sterling got dismissed similarly to Campher by Baartman, falling just 12 runs shy of scoring a century, resulting in Ireland being at 179 for 3 in the 35th over.
Harry Tector and Lorcan Tucker added 54 runs for the fifth wicket to boost the scoring rate before Tucker was caught by Ryan Rickelton off Andile Phehlukwayo for 26.
Phelukwayo got George Dockrell caught by Reeza Hendricks for six and Ireland were 243/5 in the 45th over.
Williams came back towards the end to dismiss Mark Adair (8), Fionn Hand (0), and Graham Hume (1) and ended his 10 overs with figures of 4/56, as Tector was run out for a good knock of 60 in the last over.
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Smith shines but Proteas crash to defeat against Ireland
The Proteas' chance to claim their first ODI series whitewash in 36 months may have been shot down by Ireland at the Sheikh Zayed Stadium in Abu Dhabi on Monday evening but Jason Smith's innings will have given the team's management some reason for cheer.
After their recent shock series loss to Afghanistan, Rob Walter’s men wanted to make amends with a 3-0 clean sweep over the Irish, but it was not to be on the day as they were bowled out for 215 in pursuit of 285 for victory.
The 69-run loss saw the series finish 2-1 in South Africa's favour.
Batting at number six and playing in only his second ODI, Smith scored a brilliant 91.
He started in circumspect style having failed to score in his debut but once he was settled, he pulled off an array of strokes including nine fours and four sixes in his 93 balls.
But it was not enough to stave off defeat and the victory will no doubt be celebrated from Dublin to Cork by the Irish, as it was their second over South Africa in their history in the format.
Their captain Paul Stirling led from the front and notched up an impressive 88 runs from 92 balls, which included eight fours and three sixes to contribute to Ireland's first innings total score of 284 for nine.
Graham Hume (3-29), Craig Young (3-40) and Mark Adair (2-54) played a huge role for Ireland with the ball.
The duo played a big hand in ensuring that other than Smith and Kyle Verreynne (38), Tristan Stubbs (20) and Andile Phehlukwayo (23) there was not much more to write home about for Proteas in the chase.
But Walters will take comfort in that his middle and lower order provided resilience after his top order of Ryan Rickelton (4), Reeza Hendricks (1) and Rassie van der Dussen (10) fell in quick succession leaving the side 10-3 in just the first four overs.
Earlier, Sterling and his fellow opener Andy Balbirnie (45) scored 101 runs for the first wicket before their partnership was broken by Lizaad Williams in the 23rd over.
Curtis Campher increased the run rate after coming in at No 3 but was bowled by Ottniel Baartman despite looking in good form.
Sterling got dismissed similarly to Campher by Baartman, falling just 12 runs shy of scoring a century, resulting in Ireland being at 179 for 3 in the 35th over.
Harry Tector and Lorcan Tucker added 54 runs for the fifth wicket to boost the scoring rate before Tucker was caught by Ryan Rickelton off Andile Phehlukwayo for 26.
Phelukwayo got George Dockrell caught by Reeza Hendricks for six and Ireland were 243/5 in the 45th over.
Williams came back towards the end to dismiss Mark Adair (8), Fionn Hand (0), and Graham Hume (1) and ended his 10 overs with figures of 4/56, as Tector was run out for a good knock of 60 in the last over.
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