The 17-year-old Kavanagh College pupil shocked the swimming world when she made it through to the finals of the 400m in the same stroke but was off the pace in the shorter distance.
She achieved a personal best in last night's heat to qualify 14th for the semifinals but could not improve on that, finishing last in her semifinal in 1m 59.14s. The semi was won by Australia's superstar Ariarne Titmus in 1m 54.82.
Hong Kong's Siobhan Bernadette Haughey was second and Junxuan Yang, of China, third.
The win by Titmus sets up another epic clash with Katie Ledecky in the final. The American won her semifinal, albeit in the significantly slower time of 1m 55.34s.
Fairweather will now turn her attention to the 4x200m freestyle relay - with Ali Galyer, Eve Thomas and Carina Doyle - the heats for which are tomorrow night.