"A lot of work went into that so that we were both happy."
Kirwan's decision to go will make Umaga's job - already difficult enough - slightly easier. He now has sole charge without his predecessor lurking in the background.
It won't be a quick fix, as he said, but the advantage Umaga has is an ability to get the best out of players, a track record built at Counties Manukau, whom he will coach in the ITM Cup this season, and the world champion New Zealand under-20s.
After only three victories this season, turning around the franchise's appalling results under Kirwan is Umaga's biggest challenge.
"Our winning record would probably help, but there's a lot that goes into that," he said.
"I've never been big on making bold statements and talking a lot. It will come down to our actions and that's what we need to be wary of.
"When I was growing up, the Blues were the team you always wanted to beat. They are steeped in history, they've had greats come out of this region. For me, I always thought the Blues should be the strongest.
"We have a great base here in terms of a player-base. Holding on to that is probably the biggest issue for us but hopefully we can rectify that and keep the ones we want.
"We want to be challenged, we want to get out of our comfort zones. To be the best you've got to put yourself out there and really challenge yourself and take opportunities as they come about."
Umaga will appoint two assistants to help him and current forwards coach Glenn Moore.
There will be an emphasis on developing the talent in the wider Auckland region and preventing players such as Malakai Fekitoa and Waisake Naholo leaving and flourishing elsewhere.
So, the Blues have their man, a decision unanimously approved by a previously divided board. While there was obviously an upbeat tone to the announcement, Blues chairman Tony Carter, sitting alongside Umaga, admitted there were serious issues yet to be resolved at board level - half of whom wanted Kirwan to stay and half wanted him gone.
"There are still a few personal tensions on the board, I will be the first to admit that, but we will work that through over the next months," Carter said.