Franklin's Gardens will host the huge Champions Cup Pool 1 showdown on Saturday afternoon as Saints bid to end Leinster's unbeaten start to the season.
Leo Cullen's side have won all seven of their matches in the Guinness PRO14 and both of their Champions Cup matches so far.
Saints have won four of their five Gallagher Premiership games and both European encounters.
Chris Boyd's side put local rivals Leicester Tigers to the sword last Saturday, securing an impressive 36-13 success at the Gardens.
And talented wing Larmour said: "They're a very good attacking side with a good kicking game and good defence.
"We're going to have to be switched on and to come up with a good game plan.
"This will probably be our toughest challenge yet.
"We always put pressure on ourselves to perform.
"We've got a bit of momentum now but it's just important that we forget about that, put it behind us and focus all our attention on two big weeks.
"We have high standards, and we need to live up to them.
"It's hugely exciting. Both teams have been going well. Every point counts. It's important we're switched on and we bring it.
"We're just focusing on how to beat Northampton. We'll do our homework, see where we can exploit our game plan.
"Come Saturday, the better team will win."
Leinster, who rested their first-choice stars for the win at Glasgow Warriors last weekend, are the favourites to claim Champions Cup glory this season.
And Larmour added: "We're building nicely and everyone's looking forward to getting back on the pitch this week.
"There's always different things you can work on. It's important we fix them on the training pitch.
"Last week we had about 44 people in training so there's massive competition and standards are high.
"I'm enjoying my rugby at the moment. As long as I'm getting game time, I don't really mind where I'm playing.
"You need games to be getting better. It's in the games where you really show yourself."