Savea, 30, announced Sunday he would end more than a decade of association with the high-flying Wellington Hurricanes to make the bombshell move to Moana, who finished second bottom of the Super Rugby standings in 2024.
Kirwan said the move could hamper Savea's form for the All Blacks, if Moana struggle again.
"Don't get me wrong, I want Moana Pasifika to be great. But is it good for Ardie Savea if they have another season where they are second to last?" Kirwan said on Sky Sports.
"Is that going to help us as All Blacks that Ardie is going to be coming from possibly doing extra or struggling?
"It'll be a bigger workload," added Kirwan, 59, who was basing his comments on his own experience after he signed for a second-tier club in Italy during his playing career.
Savea enjoyed a sabbatical this year playing club rugby in Japan following the 2023 World Cup.
Without him, the Hurricanes finished top of the Super Rugby table before losing in the playoff semi-finals last month.
The All Blacks back-row forward, who has played 84 Tests, will link up with older brother Julian at the Auckland-based Moana, a club comprising players with Pacific islands family backgrounds.
Savea has signed a three-year deal, leaving him eligible to represent New Zealand through to the 2027 Rugby World Cup in Australia.
Samoa-born Savea said Sunday he believed the time was right to honour his Samoan family and "give back to my heritage".
The signing of the All Blacks vice-captain is a coup for Moana, who finished bottom of the Super Rugby table in their first two seasons after joining in 2022, but has come as a disappointment to the Hurricanes.
"While we are sad to see him go, he's been a fantastic Hurricane," said Wellington chief executive Avan Lee on Monday.
"He had a clause in his contract to play for another Super team.
"The relationship with Ardie is very good. We just wish him all the best."
