The former New Zealand great told Reuters on Tuesday that the proximity of the two events could make it harder for 2019 hosts Japan to secure spending on Asia's first rugby World Cup.
"It could be a negative rather than a positive," Kirwan said. "A one-year difference is a bit tough with how the economy is.
"(But) if they're going to build five new stadiums then I'd say 'good idea' because then you can spread the infrastructure costs across two sports - which is what England is doing."
London will host the 2012 Olympics while England stages the 2015 rugby World Cup and is bidding for the 2018 and 2022 football World Cups along with Japan and several other countries.
"It's been a real positive for England because they had to spend so much money on their infrastructure," added Kirwan, who played a key role in Japan's successful 2019 bid.
"They have the Olympics, the rugby World Cup and now they're trying for the soccer World Cup - they actually pay for their infrastructure."
BUSINESS PLAN
Kirwan said Japan needed to "join the real world" and construct a self-sufficient business plan for rugby along English lines.
"They're not a mature rugby nation," he said, pointing to Japanese club side Yamaha Jubilo's decision to release its entire professional playing staff last year to cut costs.
"It's not positive. In the real world - like with English rugby and English football and New Zealand rugby - you've got to have a financial module that works."
He added: "At this stage the clubs are putting in $5 million with no return."
Kirwan wants Japanese rugby to follow soccer's winning formula after the country's successful co-hosting of the 2002 World Cup with South Korea.
"The World Cup needs to be the icing on the cake, not the cake," he said. "What you saw with the 2002 soccer World Cup is that they had 10 years of planning.
"Japan did a really good job (with the launch of the professional J-League) the World Cup was the icing on the cake. I think we (rugby) will be halfway there by 2019."
