Ulster recorded their first win on French soil thanks to a close 9-8 win over Castres at the Stade Pierre Antoine on Saturday.
With rain in the air and conditions testing, a tight, low-scoring game was anticipated between these two giants of European rugby.
And despite an early try from the hosts, there was little coming from either side in the early stages to suggest that supporters would be treated to anything else.
But, though points were few and far between, there was excitement and tension aplenty at the Stade Pierre Antoine.
Castres took the lead when Ulster spilt the ball after winning the line-out. The visiting defence initially held firm, before Yannick Forestier produced a fine pass to set up an opportunity for Remi Lamerat to score.
Crucially, the conversion was missed, and less than five minutes later Ulster had cut the deficit to two, as Pienaar stepped up to strike a penalty after Matthieu Babillot was penalised at the breakdown.
But Paddy Wallace's dangerous tackle afforded Rory Kockott the opportunity to restore the five-point cushion, before Pienaar struck his second penalty as half-time loomed to make it 8-6 at the interval.
As the teams returned for the second half, the game remained there for the taking, though it took 24 minutes following the restart for the next points to be scored - and they would prove to be crucial.
After Ulster had created an opportunity down the left, Castres were caught offside, and it again fell to Pienaar to step up and convert the chance and give his side a single-point lead.
Unsurprisingly, Castres poured forward in search of a response, but the Ulstermen held strong to secure a famous victory.
Pens: Pienaar 3
Castres: 15 Pierre Bernard, 14 Max Evans, 13 Seremaia Bai 12 Rémi Lamerat, 11 Marcel Garvey, 10 Rémi Tales, 9 Rory Kockott, 8 Pedrie Wannenburg, 7 Piula Faasalele, 6 Mathieu Babillot, 5 Christophe Samson, 4 Matthias Rolland, 3 Anton Peikrishvili, 2 Marc-Antoine Rallier, 1 Yannick Forestier.
Replacements: 16 Brice Mach, 17 Saimone Taumoepeau, 18 Mihaïta Lazar, 19 Iosefa Tekori, 20 Thierry Lacrampe, 21 Jannie Bornman, 22 Paul Bonnefond, 23 Marc Andreu.
Ulster: 15 Craig Gilroy, 14 Andrew Trimble, 13 Darren Cave, 12 Paddy Wallace, 11 Michael Allen, 10 Ruan Pienaar, 9 Paul Marshall, 8 Roger Wilson, 7 Chris Henry (c), 6 Robbie Diack, 5 Neil McComb, 4 Louis Stevenson, 3 John Afoa, 2 Rory Best, 1 Tom Court.
Replacements: 16 Rob Herring, 17 Calum Black, 18 Declan Fitzpatrick, 19 Iain Henderson, 20 Mike McComish, 21 Michael Heaney, 22 Paddy Jackson, 23 Chris Cochrane.
Referee: Nigel Owens (Wales)