The very last sentence of that article:

“Authorities have yet to confirm such a sting is what killed the two snorkelers.”

So the media, Wikipedia and hundreds of more sites get the name of the venomous jellyfish all wrong? Hard to believe:
http://en.wikipedia.org/wiki/Irukandji_jellyfish

The term irukandji is used by Department of Environment and Conservation here in Western Australia to describe the symptoms of people who have been stung by jellyfish and not the species of jellyfish itself i.e. (Irukandji Syndrome). The Scientific Editor of Ningaloo Atlas (Nick Middleton) can confirm this if you wish.
http://ningaloo-atlas.org.au/ http://ningalooatlas.org/ ph 08 9949 1805. The media got it muddled up.

The cause of death for Kathreen Ricketson and Robert Shugg is also yet to be determined and at the time of writing (end June 2013) the results of their postmortems haven’t been made public. To say they died due to jellyfish “venom” is therefore bordering on speculation. In fact there are also rumours of foul play circulating – but these rumours are also speculation until the post mortem results are made public.

One reason for the foul play rumours is because the number of jellyfish in the water in Ningaloo dropped off after April – it’s true the water temperatures were warm in Jan thru April – they always are – and there were more reported jellyfish stings than usual during this four month period. However the ocean had cooled down and returned to it’s normal cooler temperature by May – and when this incident happened the jellyfish numbers had already been waning for several weeks (in fact locals couldn’t catch any species of jellyfish even north of the site) For more information see the following article: http://ningalooatlas.org/2013/05/06/stingers-on-wane-at-ningaloo/

Another reason for the suspicion of foul play is that it’s also extremely rare for two people to be simultaneously rendered unconscious by jellyfish stings at Ningaloo. And it’s also curious that none of the other swimmers at the same beach at the same time were stung or reported seeing jellyfish.
That’s not to say it wasn’t jellyfish which left Kathreen face down unconscious – and her children definitely need our prayers and financial support – but as a rule it’s safe to snorkel at Ningaloo during the cooler water temperature months.
Warmest regards Beau

New York Daily News:
http://www.nydailynews.com/life-style/health/jellyfish-kill-australian-snorkelers-children-watch-article-1.1350988