And that would mean Worcester returning to National League One after three seasons of Premiership rugby unless they beat Sixways visitors Saracens the same afternoon.
While bonus points could still affect the relegation equation, Brain prefers to simplify matters ahead of an inevitably nerve-shredding climax.
He said: "I always had a nagging feeling it would come down to this.
"We thoroughly expect Northampton to win their last game at home to Irish - that is what we believe will happen - but it is quite simple. I believe if we win and they win, we are going to stay up."
Worcester are one point above Heineken Cup semi-finalists Saints, and they also boast a comfortably superior points difference.
Saracens, though, are going flat out for a title play-off place, whereas Irish have little to enthuse themselves after making sure of sixth place by defeating Worcester 26-16 at the Madejski Stadium.
Brain added: "Irish had far more ball and territory than we did, and control of the ball is key in warm conditions. Defending takes more out of you than attacking.
"We didn't control possession, and if you spend more time attacking than defending, then you are going to be fresher."
Saracens rugby director Alan Gaffney is taking nothing for granted ahead of the Sixways trip, when a win would guarantee his resurgent team a top-four finish.
"We are not even thinking about the play-offs yet," insisted Gaffney, after a 33-19 bonus-point victory over Harlequins.
"Worcester are playing for their lives, and that game will be a really tough ask.
"The main thing is we are in control of our own destiny. We don't have to rely on anybody else - and that is a fantastic position to be in.
"The play-off situation is simple - if we want it enough, we will achieve."
Quins rugby director Dean Richards added his seal of approval to Saracens' play-off bid, claiming: "There is no doubt they are a good side.
"Their forwards competed well and they have a lot of pace in their backs.
"They are definitely worthy of a play-off place and are up there with Leicester and Gloucester in my eyes."
Northampton, beaten 35-29 by next Sunday's Heineken semi-final opponents Wasps at Adams Park, would have already been relegated had Worcester toppled Irish and claimed a bonus point.
But they live to fight another day, although an ankle injury sustained by fly-half playmaker Carlos Spencer makes him a doubtful prospect for the European showdown.
"The game could have gone either way, but in the end we had given them too much," said Saints head coach Grayson, after his team fought back from 29-15 to draw level before being sunk by two late Mark Van Gisbergen penalties.
"The draw would have been a fair result, but it was a scrappy last five minutes."