The Reds' own whizz kid Simon Zebo grabbed a try back and the hosts cut the gap to 19-10 with a Ronan O'Gara penalty on the stroke of half-time.

In a gripping, defence-dominated second half, O'Gara kept Munster's comeback bid on course but a fourth penalty from Pienaar sent the Ulstermen through to their first European semi-final since they won the tournament back in 1999.

This was only the third ever Irish interprovincial derby in Heineken Cup history, with Munster having faced Leinster in two previous semi-finals.

Munster were hoping to pick up where they left off in January when they handed Northampton Saints a Pool 1 thrashing, but boosted by Stephen Ferris' inclusion Ulster made a purposeful start.

Amid a spine-tingling opening, ex-Ulster prop BJ Botha was penalised at the second scrum allowing Pienaar his first shot at the posts - it was a superb strike that sailed over from 54 metres out.

A rain shower did little to quell the red hot atmosphere, and Pienaar punished further Munster indiscipline with a second monster penalty in the 11th minute.

The South African's half-back partner Ian Humphreys kicked poorly out of hand early on, but collectively Ulster were quicker off the mark.

That point was underlined when Gilroy did Denis Hurley for pace on the outside, cutting in off his wing to beat tackles from Felix Jones and Lifeimi Mafi before reaching over the line for a superb 16th-minute score.

Pienaar tagged on the conversion and after Paul O'Connell was pinged for holding onto the ball on the deck, the Springbok star landed his third towering three-pointer.

Errors from Conor Murray and O'Gara did little for Munster's confidence and too often they were outfought at the breakdown, not something that often occurs on a big European day in Limerick.

Johann Muller and his fiery band of forwards drove on to set up another penalty, but Humphreys used the advantage to land a cracking drop goal from 35 metres out.

Munster needed to respond before the break and they did just that. Having sucked in the Ulster defence for once, a long accurate pass from Keith Earls gave his centre partner Mafi the opportunity to put Zebo over in the left corner.

O'Gara's successful conversion gave his side added momentum and Ulster were on the back foot after flanker Chris Henry was sin-binned for playing the ball on the ground.

The lead was down to nine points when O'Gara drove a 44-metre penalty over with the last kick of the half, rewarding an improved scrum from his forwards.

O'Gara was successful with the second of two penalty efforts, early in the second half, as James Coughlan hurtled forward on a good run.

Poor kicks from Pienaar and Andrew Trimble did little for a clearly rattled Ulster, and Pienaar's fourth penalty of the afternoon was quickly cancelled out by O'Gara.

The game developed into a slugfest between the packs as the intensity showed no sign of decreasing and Ulster's tackle count hit the three-figure mark.

A lineout steal from O'Connell kickstarted a dominant spell for Munster, yet they could not turn pressure into points as Ferris, John Afoa and tough-tackling centre Darren Cave each came up with key contributions in defence.

Staring at only their second ever Heineken Cup defeat in Limerick, Munster rallied and hunted for another game-changing moment.

Although Humphreys was inches away with a drop goal attempt and Pienaar drilled a penalty wide for his only miss of this high octane tie, Ulster hung on to claim a famous victory and a semi-final against the Scots at the Aviva Stadium on Saturday, April 28.

The quarter-final success was all the more remarkable given the fact that Brian McLaughlin did not use any replacements, and injury doubt Ferris showed immense will to last the full 80 minutes.