Despite the relegation, Cpl McNally has been in good form over the past 12 months, resulting in a call up to the England training squad.
Cpl McNally had to take time away from the game in 2017 after doctors found a hole in his heart, which was repaired with minor surgery.
On his return, he became a key player for London Irish and captained the team on several occasions.
With the move to Bath, Cpl McNally will link up with Army star Semesa Rokoduguni who has played for the professional side since 2012.
Speaking about the move, McNally expressed his joy at joining his "dream club".
He said: "For me, I think I just wanted to push myself now and really contend in the top half of the Premiership. I think Bath is the perfect place for that.
"There’s so much more to Rugby now than just going to training. I’ve got to make a decision for my career and my family. That’s something that Bath can offer.
“It’s rare that you can pick a club that ticks all your boxes.
“Bath is a great fit and they were always my number one choice. It’s a dream club for me and my family to move closer to home.
"It’s good for my family. It’s good for me personally and it’s good for the RAF too."
McNally has spent almost two and a half years at London Irish and discussed the difficulty in saying goodbye to the team.
He said: “Obviously, I was gutted to get relegated out of the Premiership last year and that’s where I want to be playing.
"I know London Irish are in a great position now and we are going to get promoted at the end of the year. I have no doubt in that.
“I’ve had lots of long discussion with Declan Kidney and it’s a decision where I didn’t really want to leave, but I have to go. He has fully supported my decision.
“Obviously, being captain now, I can see the growth of the club and the journey they are going on. It would be great to be a part of that but I think you have sometimes have to make the decision that sits right with you. That’s Bath for me.”
McNally is keen not to get to far ahead of himself though. He says he has one aim left before he leaves Irish:
“I’ve told them that I’m still Irish to until the end of the year,” he said.
"I still want to get promoted. I have a bit of a chip on my shoulder from getting relegated last year.
“I still want to lead the lads and go out to perform to the best of my ability every week to get promoted at the end of the year."