The Blues had noticed him for their under 18 ranks as either a lock or blindside flanker for St Paul's in the 1B competition. The school has a proud history of success with several 1A First XV titles in the late 1960s and 1970s.
His younger brother, Rocky, also shows promise in the league and rugby ranks.
"My background was always rugby. My grandfather (Pafelio Momoisea) played for Manu Samoa in the 1970s, so he was a big influence in getting me to play rugby. He's been pushing me all this time," says Jirah.
He was scouted by the Storm, who have strong connections in New Zealand, on his way to helping St Paul's to a second consecutive CRL premiership. They beat Wesley in the grand final and earned fourth place in last month's nationals in Papakura.
He will likely play for the Storm under 20s next year " following in the footsteps of former Auckland First XV talent such as Matt McGahan and Suliasi Vunivalu, who made Melbourne his home.
He will need to report for pre-season training by November 28, just four days after his last exam.
Momoisea learned the nuances of league swiftly, helped by his team-mates, CRL coach of the year Junior Fiu and guided by the school's director of sport, Dave McDermott.
"A lot of the boys have played league all their lives, so I've been getting guidance from them as well.
"That's been a massive help, and Junior was really big in guiding me."
Basic skills are, of course, transferrable from rugby to league for a forward.
"But, in league, you get more opportunity to run, whereas in my position in union, the breakdown means you don't have the same opportunity to show what you can do. Playing in the middle, though, is challenging. I had to get fit in the pre-season," he says.
Things got busy in terms two and three, with league games on Wednesday afternoons, First XV games on Saturday and training every other night. Sunday was a rest day.
"It was hard at the beginning. I applied for all these things and I then found I had these commitments I had to turn up to.
"My mum made me a Google calendar, jotted down all my subjects and helped me with time management."
But Momoisea's wiry frame handled most things that were thrown at him. A smart, fit player with a high work-rate, he fitted seamlessly into the St Paul's league culture.
Now he will look to slot seamlessly in to the Storm ranks.