It wasn't vintage but it was enough. And in knock-out rugby, enough is all that matters. Munster are through to a Heineken Cup semi-final against Biarritz, most likely in San Sebastian and, while they may not be in the in finest of form, there's plenty of time over the next three weeks to put that right.

"I'm relieved," said Tony McGahan afterwards. "We needed a big performance after last week and we got it." McGahan's primary emotion spoke volumes yet to say Munster weren't at the peak of their form after scoring four tries – all beautifully conceived and finished by their backs – and producing spurts of pure aggression that genuinely had Northampton reeling, merely serves to prove the incredible standard they've set for themselves over the past five years.

If you were to put your finger on it, they probably lacked the type of control over the game that they normally enjoy on home territory but maybe that's the way rugby is going – allow the opposition to dominate possession, as Munster, Leinster and Ireland have in recent times, and then hammer them into the ground when the opportunities present themselves.

Besides, it wasn't the kind of occasion for a traditional Munster mauling of an English opponent. Thomond Park, bathed in the late evening sunshine of a day more reminiscent of the south of France than Limerick, wasn't exactly an intimidating place. The province's previous victories over English opposition at this venue have been bathed in darkness and, as a result, have had a distinct air of menace about them. But how could you re-create that when the crowd were more in form for the lick of an ice-cream than the scent of blood?

But still, Munster's start was perfect. Seven seconds in and Northampton were pressurised into a knock-on, compounded the error by playing the ball in an offside position and O'Gara struck the opening points from the results penalty. A James Coughlan block-down, a couple of Tomas O'Leary garryowens and a rolling maul later, Keith Earls put Paul Warwick away in the corner and Munster were eight points in front. Northampton didn't know what hit them.

Stephen Myler kicked over two penalties to keep them hanging on but on 24 minutes they were torn apart for a second time. This time it was all down to pure Munster aggression. Jerry Flannery, Alan Quinlan, David Wallace and assorted others scraped like hungry dogs – even without the injured Paul O'Connell – at successive breakdowns inside the Northampton 22 and when the ball was eventually turned over, O'Gara released Doug Howlett on the right wing. You could say the former All Black powered past Bruce Reihana on his way over the line but, in reality, the winger reversed his way to a try much like a car into a parking space. Still, it was a nifty manoeuvre all the same.

But, credit to the visitors. Just when it seemed as though they would be washed away in a red tide, they found their feet. Myler reduced the gap to four with his third penalty on 28 minutes and, seconds before the break, they shocked Thomond Park into an unintentional silence. The Munster defence did well to put out a fire inside their 22 on the right but on the other flank a clatter of Northampton backs had gathered and when the ball was whipped across to them, Ben Foden had the simple task of putting Jon Clarke in for the try.

Myler knocked over the conversion and, puzzlingly, the visitors sat on a 16-13 lead as they sucked their oranges at half-time. But remember that creaking Munster scrum? You know, the one that was supposed to spend most of the afternoon running backwards at a rate of knots? Well, that scrum drove its Northampton counterpart towards the terrace at the Ballynanty End and, after a re-set, their point made, Coughlan fed O'Leary who found Jean de Villiers running a delicious line five yards out. The South African wasn't going to be stopped from there and with O'Gara's convert the home side were in front by four.

It was the game's crucial score. O'Gara struck two further penalties to Myler's one to inch Munster almost out of range and, on 76 minutes, Howlett made sure the home crowd could, finally, enjoy the last few rays of the evening's sun. Time for that ice-cream.