Published Monday, 25 July 2011
The 16-year old finished third in her heat of the 400 metre freestyle, which ranked her 24th overall after a time of four minutes, 16.44 seconds.
McMahon, despite it being a credible showing, was disappointed as she failed to match her time of 4.13.86, set when she won gold in the European Junior Championships a few weeks ago.
In the final, Italian Federica Pellegrini defended her world title with a comfortable win over Rebecca Adlington of Great Britain.
Irish swimmers are again in action on Monday morning when McMahon takes her place in the 100 metre breaststroke, Melanie Nocher competes in the 100 metre backstroke and Grainne Murphy in the 1500 metre freestyle.