William Porterfield???s side were set a target of 247 to secure top spot in the table following earlier triumphs over Afghanistan and Scotland, but were dismissed for 175 to lose by 71 runs.
Plenty of Ireland players made starts, with Paul Stirling top scoring with 36, but they shipped their final seven wickets for 63 runs meaning they must now beat Scotland on Monday to win the trophy.
Najibullah Zadran (83 from 50 balls) starred for Afghanistan during their total of 246-8, crunching seven fours and six sixes en route to recording his first half-century in ODI cricket.
The left-hander came in at six and put on 49 with Samiullah Shenwari (50) before dominating a 67-run stand with Afsar Zazai (18) by playing lusty blows against spinner George Dockrell and medium-pacer Alex Cusack.
Asghar Stanikzai (44) and Shenwari had earlier combined to add 61 for the third wicket after Craig Young had reduced Afghanistan to 38-2 in the seventh over.
Young and fellow seamer Kevin O???Brien collected three wickets each, with the latter forcing Zadran to miscue a pull and hole out to short mid-wicket.
Stirling got Ireland???s run chase off to a brisk start but neither he nor his team-mates could build after getting themselves in.
Ed Joyce (21), Niall O???Brien (33), Gary Wilson (20), Kevin O???Brien (21) and Andrew Balbirnie (17) came and went, before Afghanistan quickly polished off Ireland???s tail, with Hamid Hassan ending with figures of 3-24.
Afghanistan will be tied with Ireland and Scotland on four points if the latter defeat the former on Monday but cannot win the Triangular Series due to their negative run rate.