Chasing Ireland's 248 for eight, Ryder made his intentions clear from the start, sending four balls to the fence in Craig Young's opening over.
The third over of Otago's chase was the most punishing, with Ryder taking 22 off it, including two sixes and a couple of fours.
He raced to 51 off 16 balls, a feat that would have made it the third fastest half-century in history, had it been a list A match.
The fastest ODI 50 was hit by Sri Lanka's Sanath Jayasuriya (17 balls) against Pakistan in 1996. Brendon McCullum (19 balls) has the fastest by a New Zealander.
Despite losing team-mates Ryan Duffy, Michael Bracewell and Aaron Redmond inside the first seven overs, Ryder did not let up.
He smacked 72 of Otago's opening 80 runs and was particularly harsh on Young, whose four overs went for 51 runs.
The opener's ton came off 39 balls, which would have been the fourth-fastest list A century had it counted. Cory Anderson (36 balls) set the record against the West Indies in Queenstown earlier this year.
When Ryder was eventually caught off George Dockrell's bowling for 136, he had tallied 18 fours and eight sixes at a strike rate of 238.59.
At 186 for four after 17.4 overs, Ryder's work was done. Iain Robertson, who hit 51 from 40 balls against Scotland yesterday, notched up another half-century, scoring 53 from 50 balls. Robertson and Ryder put on 106 runs for the fourth wicket in 10.3 overs.
Sam Wells (20) and Nick Beard (24) ensured Otago made it home, doing so by three wickets and with 18.1 overs left.
Earlier, opener Andy Balbirnie scored 59 off 80 in Ireland's innings, while Andy McBrine hit 43 off 41 batting at No 10.
Beard and Roald Badenhorst took two wickets apiece for Otago and Sam Wells took one wicket for 28 runs off nine overs.
-by Robert Van Royen
