As the RaboDirect PRO12 campaign moves towards a gripping finish, Glasgow Warriors are in position to go one better than last season, all the way to the final.
If Gregor Townsend's team is to break the Irish-Welsh duopoly which has lasted since the first running of what was then the Celtic League 11 years ago, they must see off the two most successful teams in the history of the competition.
To ensure they remain on course for a home semi-final, the second-placed Warriors will be braced for two demanding examinations of their capacity to win the big ones.
Triple champions Munster are first up at Scotstoun Stadium on Friday night. The former champions, whose most recent title two seasons ago came at Leinster's expense a week after they had won the European Cup, dare not lose if they are to have any hope of wiping out the seven-point gap separating them from the play-off zone.
They have won their last six RaboDirect PRO12 matches against the Scots, helped on their way in four of those fixtures with a grand total of 26 goals from Ronan O'Gara.
The Warriors, the highest try scorers in the tournament with 49, one more than Leinster and five more than Ulster, at least have the law of averages on their side. Nothing less than a home win will be good enough to keep them in the top two.
In their last home match of the regular campaign, they go head-to-head with the Welsh region whose victory in last year's Grand Final in Dublin gave them the trophy for a record fourth time.
The Ospreys are old hands when it comes to qualifying for the play-off system now in its fourth season.
During that time they have achieved the distinction of winning two Grand Finals, beating Leinster in their own backyard at the RDS in 2010 and again last season.
They did it in some style, too, Shane Williams bringing the curtain down on his Welsh career with the latest of tries which still required a touchline conversion from Dan Biggar.
The Wales fly-half sent it steepling between the uprights to clinch the narrowest of victories, 31-30.
Townsend knows to his cost that beating Leinster in Dublin takes some doing.
The Warriors, 17-6 ahead before half-time in last week's match, had every reason to believe they would hold on for a record seventh straight win.
They were still in front with eight minutes left when Ian Madigan's try ensured a priceless 22-17 home win, the fly-half converting it himself to account for every single Leinster point.
It may yet prove to be a rehearsal of the Grand Final, not that Joe Schmidt, for one, sounds at all keen on the idea.
"They are a very good team," Leinster's Kiwi head coach said of the Warriors. "I would be delighted if we don't see them again this season."
Their dream of three straight European titles broken by those successive defeats by Clermont on successive weekends before Christmas, Leinster face a colossal all-Irish affair on Saturday against familiar opponents from across the border.
Ulster, engulfed by Leinster's exhilarating brand of Total Rugby during last season's European final at Twickenham, go to Dublin after dropping out of the top two for the first time all season.
Three defeats in their last four matches - against Ospreys, Warriors and Edinburgh - have pushed them down to third.
While the Irish provinces fight it out for top spot, the two Welsh play-off contenders will be otherwise engaged at the Millennium Stadium's first double-header of its kind. 'Judgement Day' pits the Ospreys against the Blues - Justin Tipuric against Sam Warburton.
The Blues, beaten in three of their last four matches, lost 33-12 in Swansea last November and, just to prove they are nothing if not consistent, they lost to their neighbours by exactly the same score at the Cardiff City Stadium last season.
The Scarlets, squeezed out of the top four by their local rivals last weekend, meet a Dragons team anxious to stop the rot after conceding seven tries to the Ospreys in Swansea last Friday night.
One Welsh team ought to make the semi-finals in May. Which one is another matter. Three of the Ospreys' last four fixtures are away, three of the Scarlets' last four are at home.
The top four and their run-in over the last four matches:
Leinster (1st, 63 pts):
v Ulster (h), Munster (a), Zebre (a), Ospreys (h)
Glasgow Warriors (2nd, 62 pts)
v Munster (h), Scarlets (a), Ospreys (h), Connacht (a)
Ulster (3rd, 62 pts):
v Leinster (a), Dragons (h), Connacht (a), Blues (h).
Ospreys (4th, 54 pts):
v Blues (a), Treviso (h), Glasgow Warriors (a), Leinster (a).
Scarlets (5th, 54 pts):
v Dragons (a), Glasgow Warriors (h), Blues (h), Treviso (h).
Munster (6th, 47 pts):
v Glasgow Warriors (a), Leinster (h), Dragons (a), Zebre (a).
Play-offs last season:
Semi-finals:
Leinster 19, Glasgow Warriors 15. Ospreys 45, Munster 10.
Final: Leinster 30 pts, Ospreys 31.
Season 2010-11:
Semi-finals:
Munster 18, Ospreys 11. Leinster 18, Ulster 3.
Final: Munster 17, Leinster 9.
Season 2009-10:
Semi-finals: Leinster 16, Munster 6. Ospreys 20, Glasgow 5.
Final: Leinster 12, Ospreys 17.
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