Michael O'Neill's side were 10 minutes away from a famous victory over a Portugal side 114 places above them in the FIFA rankings, but still managed to hold out for a famous point.
The Group F match was supposed to be all about the Real Madrid superstar, who was presented with his 100th cap in a pre-match ceremony involving Luis Figo, but O'Neill's men took a shock lead on the half-hour when Niall McGinn capped a fine counter-attack.
Ronaldo, and his illustrious team-mates, dominated possession and chances both before and after that but had to wait until the 80th minute for Helder Postiga to turn home a scrappy equaliser.
A point will be celebrated by a Northern Ireland side who were held at home by lowly Luxembourg last time out, but their bravery in the face of an attacking onslaught from Ronaldo and company so nearly yielded even more.
O'Neill sent out a compact 4-5-1 formation, resisting the temptation to start with Dean Shiels as a link between midfield and attack.
With an absentee list in double figures, there were recalls for Corry Evans, Oliver Norwood and McGinn.
The kick-off was slightly delayed as Ronaldo received a special presentation from fellow centurion Figo but when play did get under way it took him just two minutes to register his first shot.
The 25-yard effort was hardly befitting of the occasion, a scuffed attempt that rolled harmlessly wide.
Ronaldo clearly had goals on his mind and sent a second shot wide in the fifth minute.
The home skipper was not playing a lone hand though, with Nani and Joao Moutinho both looking to target Ryan McGivern at left-back.
But Jonny Evans looked in determined mood at the back, twice clearing the danger as Portugal closed in.
Ronaldo, seeking a change in fortunes, moved from left wing to right and drilled a dangerous cross into the area only for Craig Cathcart to clear.
With Northern Ireland struggling for meaningful possession in the final third Evans took matters into his own hands, attempting to catch Patricio off his line with a free-kick placed 10 yards inside his own area.
The idea was Ronaldo-esque, though the execution fell some way short.
Then, with half an hour gone, the Porto crowd was stunned into silence.
Evans launched the ball up front and Kyle Lafferty did well to get it down to his feet and under control.
The Portuguese defence, pressing up the pitch, were caught napping and when McGinn raced forward into space, Lafferty found him with the perfect pass.
The Aberdeen winger was one on one with Patricio and calmly lifted his shot over the goalkeeper before celebrating wildly with the travelling supporters.
Ronaldo was sure he had equalised in the 35th minute when he poked a boot at Joao Pereira's cross but the ball cannoned back off the bar.
Ronaldo, Nani and Pereira were crossing almost at will from the right but the visitors, marshalled by the excellent Evans, kept clearing the danger to maintain their shock lead at the interval.
The second half began in much the same fashion as the first had ended, Portugal dominating the ball only to be foiled as they moved in on goal.
Ruben Micael attempted to change the pattern with a long-range shot soon after the restart but cleared both Roy Carroll and the bar.
With 55 minutes gone Portugal, and Ronaldo, thought they had finally made the breakthrough.
Miguel Veloso floated a pass to the Real forward at the back post but his header, from just six yards, was directed straight at Carroll.
The 35-year-old, who was unhappy about lasers being shone in his eyes, made a more impressive save from Ronaldo moments later, getting down well to beat away a first-time effort.
By now Northern Ireland were pinned in their own box, Nani almost coaxing a penalty from McGivern and Steven Davis nearly diverting the ball into his own net.
Portugal, meanwhile, were becoming visibly frustrated as the unlikely shutout continued.
With 20 minutes remaining Ronaldo appeared to break free of the defence but, just as he lined up his shot, Aaron Hughes did enough to deflect it wide for a corner.
McGinn mis-hit when presented with a rare opportunity at the other end, Norwood having found him with a clever pass.
The equaliser belatedly arrived in the 80th minute, the previously off-colour Postiga stabbing home in a goalmouth scramble.
Substitute Eder nodded a deep cross back across goal, with Nani touching it on to the striker, who picked himself up from an awkward position to prod past Carroll.
Varela and Nani both went close to a winner inside 90 minutes, while Eder was millimetres away from turning home in an overly generous period of added time.