Northern Ireland came agonisingly close to a shock victory in Porto on Tuesday after taking the lead through McGinn in the first half.
The Aberdeen midfielder showed superb composure to beat Rui Patricio with a lofted finish for his maiden strike at international level.
Portugal dominated the second half and found an equaliser with 10 minutes to play through Helder Postiga.
"It was a dream come true to score. I think the boys were first class for 90 minutes and we a are a little bit disappointed to give away the late goal," the 25-year-old told Sky Sports News.
"We have to credit ourselves for the performance tonight and from a personal point of view I'm over the moon to get my first international goal."
Martin O'Neill's men have collected two points from their opening three World Cup qualifiers but can look forward to four consecutive home matches, starting against Azerbaijan in November.