Farrell redeemed himself by converting a 79th minute penalty for a losing bonus point which keeps Saracens at the top of Pool 1, albeit with Munster now level on 10 points ahead of next week's return fixture in Watford.

This absorbing encounter was balanced on a knife edge throughout, with industrious Munster gaining the early edge to take a 9-3 interval lead through O'Gara.

The immense physicality spilled over at times, leading to yellow cards for Donncha O'Callaghan and Rhys Gills, while Saracens' lineout creased alarmingly under pressure from the hosts.

With man-of-the-match James Coughlan producing the goods up front, O'Gara hammered over two more kicks to stretch the lead to nine points.

But Farrell had the final say as the England international, who was only 14 when Munster won their first Heineken Cup, coolly completed his penalty hat-trick in the dying minutes.

Saracens welcomed back four of England's All Black slayers into their starting line-up - Alex Goode, Chris Ashton, Brad Barritt and IRB Player of the Year nominee Farrell.

Amid a back row injury crisis, Munster handed a full Heineken Cup debut to Cork youngster David O'Callaghan while a back problem continues to rule out former captain Paul O'Connell.

Stumbling Sarries let O'Gara's kick-off bounce before Ashton was enveloped by a robust red wave, the visitors steadying themselves with one of their supposed strongest facets - the lineout.

Munster set the early intensity though, Will Fraser being caught offside and O'Gara confidently knocked over the left-sided penalty in the fifth minute for the lead score.

A neatly-worked lineout move put Conor Murray dummying past both Schalk Brits and Charlie Hodgson, however an awkward final pass was spilled by the supporting Simon Zebo.

As the game took off, Ashton almost intercepted a looping pass as Munster continued to move the ball wide, keeping to the expansive style favoured by head coach Rob Penney.

The first sign of a try-scoring threat from the Englishmen was visible in the 12th minute. Goode countered from deep and linked with Barritt and Ashton with the latter's dinked kick grounded just in time by the retreating Felix Jones.

Farrell failed with his first penalty attempt after Mike Sherry went off his feet at a midfield ruck, and that rarity of things - a loose Sarries lineout - saw O'Gara drill Munster back downfield.

French referee Pascal Gauzere did not endear himself to the Thomond Park faithful, dishing out six penalties against Munster of the opening nine.

The first flashpoint, midway through the half, resulted in Gauzere flashing yellow cards to O'Callaghan and Gill for an off-the-ball dust-up that led to others piling in.

Munster exerted pressure on Saracens' lineout again and turnover ball paved the way for O'Gara's second penalty goal, following good carries from Coughlan and David O'Callaghan.

Brits was chopped down by a thumping tackle from Munster skipper Doug Howlett, but Saracens looked increasingly dangerous as Barritt and Farrell tried to unlock the home defence.

The sides swapped penalties coming up to half-time, with Farrell landing a three-pointer after a scrum infringement and O'Gara punishing Ernst Joubert for interfering with Donncha O'Callaghan in a lineout.

O'Gara, the tournament's record points scorer with over 1300 points, drifted a drop goal wide in the last act of a tense and feverish first half.

Saracens made the stronger start to the second period, benefiting from some further indiscipline at the breakdown from Munster. A 37-metre penalty from Farrell increased the heat on Penney's men.

The home forwards produced a storming response, winning a scrum against the head before O'Gara fired over his fourth surefooted strike from inside the Sarries' 22.

A fifth followed as Munster began to dominate in the possession stakes, with Keith Earls and James Downey probing in the middle and the back row working ferociously as a unit.

Saracens were living off Munster mistakes and Farrell drifted a 44-metre penalty to the left and wide as the drizzle came down.

Mark McCall sent for reinforcements and Richard Wigglesworth helped Sarries finally build through a series of phases. Flanker Fraser was just denied in the right corner by a combination of Zebo and Murray.

There was glimpse of Farrell's talent in the build-up with a deft flicked pass to Goode creating the space for Fraser's opportunity. Yet, Munster held out and they rebounded for an edgy final ten minutes.

Howlett and Ashton both threatened on their wings, the latter's touchline dart earning a 75th minute penalty which Farrell sent wide off the left hand upright.

But O'Gara blundered as he pushed the resulting drop-out straight into touch. Sarries steeled themselves for a final charge and the fruits of their labour could yet prove vital when the pool placings are decided next month.