The former Australia centre, who won 34 caps from 2010, scored two tries as Saints racked up six touchdowns in all and had the game wrapped up by the break.
Horne formed a hard-hitting midfield with Luther Burrell until the latter was taken off with a knee injury after 63 minutes.
The win pushed Northampton up to third in the table just three weeks after the embarrassment of their 55-24 collapse to Saracens at Twickenham.
And Horne, who missed that miserable day, was at the centre of it all with his two scores when he seemed to run over half of the Irish team and brought valuable abrasiveness to the midfield.
Director of rugby Jim Mallinder said: “Today he was outstanding – tough as nails. He was always going to score those two tries, he saw the line and he wasn’t going to be stopped. He has given us that bit of composure in defence and the punch when he has to make a tackle.”
The writing was on the wall for Irish after just 92 seconds when Northampton scrum-half Nic Groom scooted over after a break by George North had seen Burrell and lock Michael Paterson combine to put the South African away.
And it was pretty much one-way traffic until Saints had got the bonus point when hooker Mikey Haywood, in for the injured Dylan Hartley, made the most of a driving lineout on 33 minutes.
Horne’s two scores were sandwiched between that pair and both involved powerful, straight-up running and the ability to crash over defenders.
Mallinder added: “That start was just we talked about – being direct and being physical.”
Newly-promoted Irish have now conceded at least five tries in their last three games – against Exeter, Sale and here – which has burst the balloon after the win over Harlequins in round one.
Once they woke up, when Northampton were home and hosed, they played a bit of rugby and wing Alex Lewington scored a memorable 40m try when he beat North, and Tommy Bell was put away for another by a good long pass from Blair Cowan.
But the game had long gone by the time those second-half tries came and North’s run-in put the seal on it before flanker Josh McNally’s score.
Director of rugby Nick Kennedy said: “For the third week in a row we gave ourselves a mountain to climb against a very good team. They came out firing and scored a couple of decent tries.
“We have conceded a lot of points but we were aware how hard it was going to be.”
London Irish – Tries: Lewington, Bell, McNally. Cons: Bell, Tonks. Pens: Bell 2.
Northampton – Tries: Groom, Horne 2, Haywood, North, Reinach. Cons: Mallinder 5.
Source : EXPRESS