Porterfield, in the form of his life, will be disappointed he popped a return catch on 84 when a second century in eight days was there for the taking but Niall O’Brien’s 64 not out took Ireland over the line with five overs to spare.
The south-west Africans had proved doughty opponents in two wonderful InterContinental Cup matches before Christmas but they were never in contention here after winning the toss and slipping to 66 for six.
Boyd Rankin produced another excellent opening spell but it was Alex Cusack who really rocked Namibia with an amazing run and dive to catch danger man Gerrie Snyman on the leg-side boundary.
The most disciplined performance in the field so far from Ireland — including a direct hit run out by Andrew White — kept Namibia in check until Nicolaas Scholtz rallied the tail to reach a respectable if never threatening 213 for nine.
Even that total would have been unlikely if Ian van Zyl had not enjoyed an extraordinary slice of luck in being given not out by both umpires when evidence suggested he was both caught behind and stumped off the same ball from White.
A stunning piece of fielding from John Mooney removed Scholtz for a top score of 62 in the final over. The left-hander thought he had struck a six but Mooney leapt at long on to knock the ball back onto the field and then hurled it to the keeper with Scholtz stranded.
The Irish reply began badly when Gary Wilson was yorked for a duck in the second over but there were no further blemishes until Eoin Morgan holed out on the deep midwicket boundary for 41.
By now, Namibia were more interested in whether they might sneak into the Super Eight on the back of Oman beating Uganda — they did — and there was little tension as Ireland eased towards their target and reached it with a Kevin O’Brien boundary for the third time in as many games.
Porterfield, whose side will play Afghanistan, the UAE, Holland and Kenya in the next eight days, said: “We’ve achieved our aim of winning all five Group games and taking three wins through with us into the Super Eight.”
At WITS University, Johannesburg (Namibia won toss): NAMIBIA 213-9 (50 overs) (N Scholtz 62, B Rankin 2-17) lost to IRELAND 214-3 (45 overs) (W Porterfield 84, N O’Brien 64no, E Morgan 41) by seven wickets.