Despite that setback, Cox showed her characteristic determination to bounce back.
Other top 10 placings came in the 400m freestyle (6th) and the 100m backstroke (9th).
John Paul College student Irving 13, underlined her rapid progress over the last 12 months with a solid showing in six events.
Irving produced a fabulous 100m freestyle to drop her best time again this year. The clock was stopped at a swift 1m 08.86s.
Irving backed that result up with 1m 8s splits in the Bay of Plenty regional relays.
She blitzed her heat in the 50m backstroke to post another personal best time by almost 4s with an excellent time of 36.90s.
Head coach Alastair Johnson was pleased with the performances.
"Both Kaira and Samantha have shown great commitment over the last year. As a result their performances have improved beyond all recognition.
This was their first experience of competing at a national event, and both did a really good job which will no doubt encourage them to push on even more.
"To me both Kaira and Samantha personify our new club slogan of 'Swim Rotorua - where swimmers excel.'"