Leinster became the first team to lose a home Pro12 semi-final when they failed to break down Scarlets at the RDS despite having an extra man for the entire second half, with the Welsh side running out deserved 27-15 winners.
The Welsh visitors stunned the crowd in Dublin by outscoring their hosts three tries to one in the first half to build a 21-10 interval lead.
Leinster were reeling but looked to have been given a lifeline when Evans was shown a red card just before half-time for a tip-tackle on Ringrose.
In the second half they were repeatedly undone by loose kicking and knocking on or conceding penalties when in Scarlets territory.
Although they did close the gap when Jack Conan barged over just after the hour mark, captain Isa Nacewa kicked the conversion off the post from close range, which meant another try was required.
That never came as Leinster grew visibly more frustrated and ragged and Liam Williams clinched a stunning victory for Scarlets with two late penalties.
The manner of this defeat - the first for a home team in 19 PRO12 semi-finals - will sting Leo Cullen's men over the summer months, especially coming on the back of their recent Champions Cup last-four exit
Leinster captain Nacewa had a try ruled out for a forward pass from Jonathan Sexton amid a dominant opening from the province.
However, it was Scarlets who nipped ahead in the ninth minute, Evans scampering over for his 12th try of the campaign after Adam Byrne's missed tackle had allowed Jonathan Davies to tee up a classy converted score.
Ringrose was Leinster's bright spark, but four handling errors inside the first 14 minutes left the hosts frustrated. They got off the mark five minutes later, Nacewa's penalty punishing Lewis Rawlins for not releasing after the tackle.
Evans' subsequent try-saving tackle on Luke McGrath, combined with Liam Williams' poaching skills, denied the hosts soon after, with McGrath suffering a game-ending injury in the process.
Leinster's replacement scrum-half Jamison Gibson Park made an immediate impact, inspiring some excellent interplay off a lineout which ended with Conan putting Ringrose over for his 23rd-minute try.
But Scarlets' attack oozed quality off limited ball, Gibson Park's partially-blocked kick providing the platform from which Shingler exploited a gap and linked with scrum-half Davies for a sucker-punch score.
Leinster's defence lulled again on the half hour as brilliant hands from Scott Williams, man-of-the-match James Davies and Evans played in number nine Davies for a third converted effort - Rhys Patchell supplied the extras again.
A couple of flash-points, including a scuffle involving sparked by a shoulder from Gareth Davies on Sexton, increased the decibel level, and Evans' lifting of Ringrose in the tackle left his side down to 14 men.
In truth, Evans' team-mate Samson Lee could have been the one to walk either as they both combined to drive the Irishman head-first into the ground.
Individual mistakes prevented Leinster from exploiting their numerical advantage on the resumption, the visitors surviving the third quarter unscathed with James Davies, who covered on the wing, and former Leinster lock Tadhg Beirne both impressing.
Ryan Elias' overthrown lineout saw Scarlets slip up as Conan managed to barge over from close range.
Nacewa fluffed his lines, though, with a howler of a conversion, and it was Wales' first PRO12 finalists since 2012 who held their nerve.
With Patchell replaced, winger Williams split the posts in the 70th and 80th minutes, punishing Cian Healy for a scrum offence and Ross Molony for a high tackle.
Scarlets will now face Munster or Ospreys in the final at the Aviva Stadium next weekend as Leinster are left to reflect on a league campaign that ends at the semi-final stage despite them leading the table up until the final day of the season.