Birchall's manager Shane Casley, of TLA Australia, revealed the news on Trade Radio on Thursday.
"The plan is to get him up there for a 2-3 year period, and Grant thinks he has that much footy left in him,'' he said.
"I think the main thing for Grant was to make sure his knee was good and now he's confident he has good footy in him for the next two to three years.
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"Grant identified Brisbane as a team on the rise and that was a big reason why he wanted to go."
Birchall, the four-time premiership Hawk from Devonport, will join fellow Tasmanian, and former Hawthorn footy boss Chris Fagan, at the Lions, where he will replace his former Hawthorn teammate Luke Hodge in defence.
He'd had offers from Hawthorn (which had earlier said it would wait until after the trade period to make its decision on his future), Brisbane and Gold Coast.
The 14th pick in the 2005 national draft has been restricted to just eight games in the past three years due to knee and hamstring and a broken jaw.
Hawthorn general manager of football Graham Wright said his fellow Tasmanian left the Hawks with their full support.
"We were always open with Birch and his management about the fact we would be unable to guarantee him a list spot until we got through the trade period and had a better idea of where our list sat," he said.
"As an unrestricted free agent Birch exercised his right to explore the market and we respect that completely.
"The last couple of seasons have been incredibly frustrating for Birch on the injury front and we are pleased for him that the Lions are able to provide him the opportunity to continue his playing career.
"Birch has been a fantastic servant of our footy club for 14 years and we wish him all the best in the next chapter with the Lions."
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