The Baby Boks had prevailed 42-26 in the side’s previous meeting in the pool stages but Ireland had competed gamely and were in contention up until the last 10 minutes.
However, they were out of this particular contest by the interval, with the South Africans racking up five first-half tries through Cornell Du Preez, Wandile Mjekevu, who struck twice, Arno Botha and Tshotsho Mbovane.
Ireland had shown impressive fitness levels in disposing of Scotland in Rovigo on Saturday to secure their place in the play-offs for fifth to eighth place, running in three tries in the final 10 minutes.
However, they looked lethargic last night, conceding a try after just three minutes, flanker Du Preez bounding over from under the posts after reacting first to a loose pass from centre Francois Venter.
They were over again six minutes later, Mjekevu racing away to score after pouncing on a turnover on halfway by Ireland full-back Tiernan O’Halloran.
Ireland tried to rally and enjoyed a decent spell of possession soon after but their play was blighted by poor kicking and knock-ons. They were also struggling to make their tackles stick, illustrated by the failure of both Paddy Jackson and Andrew Conway to halt South African captain Botha as he charged through for his side’s third try on 14 minutes.
James McKinney belatedly got Ireland off the mark with a penalty but South Africa responded with two tries in three minutes before the half hour.
Mjekevu scored the first, the winger left with another simple finish after O’Halloran spilled a pass from centre Brendan Macken in midfield, while the second went to Mbovane, who showed good pace as he finished off a fine cross-field move.
The final 10 minutes of the half provided Ireland with some respite but South Africa registered their sixth try within two minutes of the second half, outside centre Jaco Taute breaking through some more weak tackles to score in the left corner.
Ireland had no response to South Africa’s energy and powerful running, and Mbovane helped himself to a second try on 55 minutes after being released by replacement Mbongeni Mbonambi.
Ireland finally managed to get themselves over the whitewash midway through the second half when Conor Carey and Paddy Jackson combined to put No. 6 Eoin McKeon over in the right corner.
South Africa refused to ease up in the final quarter, though, and their superiority was underlined by further tries from inside centre Venter and Mbovane.
Ireland did keep battling until the death and their fighting spirit was rewarded when fly-half Jackson broke through to score with just over two minutes to go.
T O’Halloran; A Conway, B Macken (J McKinney, 47), L Marshall (A Kelly, 45), A Boyle; P Jackson, K Marmion; J Tracy (F Bealham, 45), N Annett, T Furlong (C Carey, 45); M Kearney (D Qualter, 45), I Henderson; E McKeon, D Doyle, J Murphy (S Buckley, 50).
U Beyers (P Jordaan, 70); T Mbovane, J Taute, F Venter, W Mjekevu; J Goosen (J Welthagen, 55), P Rademan (L Jacobs, 63); S Kotze (N Schonert, 69), M Van Vuuren (M Mbonambi, 40), N Schonert (J R Jenkinson, 40); J Cook, E Etzebeth; F Kleinhans (N Carr, 71), C Du Preez, A Botha.
Referee: F Pastrana (Argentina).