Ed Joyce and Wilson kept Ireland on target for a win with a stand of 91.
Fellow left-hander Ed Joyce also made his way to 38 not out off 57 balls.
But Ed Joyce's golden week turned sour as the victim of a freak dismissal.
Sussex lost acting skipper Ed Joyce first ball in the opening over and could only muster 95-8.
Du Plessis swiftly followed when he was undone by extra bounce and spooned a catch to Ed Joyce at cover.
Eoin Morgan and Ed Joyce are examples of Irish players who have gone on to represent England in recent years.
And Ed Joyce then lost his off-stump to Calum Thorp's inswinger in the sixth to leave Sussex on 16 for two.
After Murray Goodwin (93) and Ed Joyce (74) had laid the foundations, captain Mike Yardy saw his side home on 288-4.
Batsman Ed Joyce is also in the panel, four years after playing for England against the Irish in the West Indies.
BBC: Cricket World Cup: Regan West left out of Ireland squad
England will be without the injured Andrew Flintoff but Dublin-born Ed Joyce is set to make his debut against his native country.
Ed Joyce and Niall O'Brien put on a quick 54 and Gary Wilson kept up the momentum with 27 from 21 balls.
Lee took 2-30 from his four overs, marking his debut in satisfactory style by getting rid of openers Rory Hamilton-Brown and Ed Joyce.
Seam bowlers Chris Whelan and Gareth Andrew made a double breakthrough and Ed Joyce tamely smacked the last ball of Fisher's first over to mid-on.
Sussex Sharks had set Nottinghamshire a target of 255 for victory with Rory Hamilton-Brown (49) and Ed Joyce (42) top scoring for the league leaders.
Niall O'Brien and Ed Joyce rode their luck in seeing Ireland to 103-2 before a devastating burst from Graeme Swann put England in total control.
Following Weston's dismissal, Irishman Ed Joyce inflicted further punishment on the tourists, making 80 before falling to a run-out in the closing overs of the day.
At that stage Middlesex had crumpled to 34 for four but Ed Joyce continued his recent good form as he and Jamie Dalrymple rebuilt the innings.
Alleyne showed his all-round talents to remove Ed Joyce for 26, and though Weekes kept the innings alive with a five-boundary 53 Middlesex were always behind schedule.
Ed Joyce and captain Michael Yardy - back after missing his team's first match against New South Wales because of an illness - briefly held the innings together.
Ed Joyce's century was the highlight of an eight-wicket win at Durham, before Mike Yardy's side survived a late Surrey assault at Hove to get home by two runs.
Middlesex slipped to 114-4 but Ed Joyce shared an unbroken 39 with Dawid Malan to see them home with an over to spare as Sussex slipped to a seventh defeat.
Ed Joyce, a member of the England team during the 2007 World Cup, is set to face his former side at M Chinnaswamy Stadium in Banglaore with a natty cut and dye.
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