Ulster earned a home semi-final against the Scarlets by seeing off Cardiff Blues at a wet Ravenhill to finish top of the RaboDirect PRO12 table.
Two second half tries from Andrew Trimble and a late score for 100 cap man Robbie Diack helped Ulster on their way after Mark Anscombe's side had trailed 13-10 at half-time against a Blues effort which petered out in the second half.
It started badly for the hosts when Dafydd Hewitt scored by the posts after three minutes following a break from Lewis Jones when the scrum half broke inside Pienaar. Rhys Patchell added the simple conversion and the Blues were up and running.
Pienaar was then wide with a straightforward penalty three minutes later but made amends after Harry Robinson dropped Hewitt's pass in the Blues 22, allowing Pienaar to collect and score. He added the extras to tie the scores after nine minutes.
Ten minutes later though a Patchell penalty nudged the Blues in front again and then Ulster lost John Afoa to a hamstring injury, though a Pienaar after 24 minutes tied the scores again.
But the Blues got back in front again shortly afterwards when Patchell kicked a neat drop goal and there were no further scores in a scrappy remainder of the half with the visitors leading 13-10.
Pienaar converted and then added a long range penalty shortly afterwards to give his side a 20-13 lead.
This was then increased to a 10-point lead after Patchell spilt a Pienaar Garryowen and the Blues conceded a penalty.
With the rain now falling heavily, Patchell was wide with a penalty attempt to get the Blues back in the game and Ulster immediately punished his profligacy when a mix-up at the back saw Trimble race on to grab his second try of the night.
Pienaar superbly converted and Ulster now looked secure and out of sight at 30-13 and with both benches beginning to empty the game became even more fractured and scrappy.
Ulster bungled a number of scoring chances and then with Hewitt sent to the bin, Diack charged over at the posts which Pienaar again converted with three minutes remaining.