There are also three Magners League teams, including Connacht, in the last-eight of this season's Amlin Challenge Cup.
But while reigning champions Leinster, Munster and the Ospreys will be hoping to take a step closer to the Heineken Cup final in Paris on May 22, and Connacht, the Cardiff Blues and the Scarlets chase glory in the Challenge Cup, nobody is forgetting their biggest domestic date.
On Saturday, May 29, the Magners League title will be decided at the grand final and the race for a place in the final four could not be keener as the countdown clock reaches 50 days to go.
With the top four teams qualifying for the play-offs this season, the semi-finals would look like this with three rounds left to play - Leinster v Glasgow Warriors and Munster v Edinburgh.
But the Ospreys, the Newport Gwent Dragons and even Cardiff in seventh place have not given up on the chance of reaching the top four before the season ends.
Munster captain Paul O'Connell said: "The standard of the Magners League has improved dramatically. You have seen that over the last few years with teams reaching quarter and semi-finals in Europe.
"In the past, when the Magners League was a smaller competition, we found it difficult to cope with some of the teams in the latter stages of Europe who were more used to playing at that level week in, week out.
"Having played that Magners League game against Leinster last weekend will have been good for us going into the Heineken Cup quarter-finals.
"We played in a 26,000 stadium that was full to capacity and there was massive media interest around the fixture."
He added: "The play-offs are a great thing for the Magners League and there is a big battle on for those top four positions. That makes the Magners League a great competition.
"If you can be in the knock-out stages of the Magners League and the Heineken Cup, you can look forward to playing in big games in front of big crowds in good conditions."
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