An 18-11 win over the Ospreys saw Munster secure a showdown with Leinster at Thomond Park on May 28.
And O'Connell, who has played 11 times for the Lions, reckons the game will be of huge significance for the whole of Irish rugby.
"It's going to be very tough," said O'Connell. "Leinster are a good side and are playing really well. But we will be at home and have to protect that. It's going to be a huge day for Irish rugby."
The colossal second-row was delighted as his team ground out the victory over the defending champions, who boasted five former Lions in their match-day squad.
A try in either half from centre Danny Barnes paved the way to glory and O'Connell was pleased to see his side make amends for their Amlin Cup semi-final exit.
"We needed a performance," said O'Connell. "We've struggled in the last few games and needed to put in a performance and get a result.
"We were very disappointed with the semi-final loss two weeks ago. Sometimes you get caught up on trying to play the perfect game
"We probably left a few tries out there but we played well and put them under pressure.
"We were very patient in the first half that probably went in the second and we tried to force things a bit.
"But we have struggled against the Ospreys in the past so to put in a performance and dominate them as much as we did was great.
"We just needed to keep the scoreboard ticking over. We would've liked to have gone over for one or two more ties but our heads didn't drop and we kept at it."
O'Connell was particularly happy with the Munster scrum, an area the Ospreys have dominated in the previous meeting between the two sides.
"We struggled against them at times this year but our scrum did well and out lineout was good," added O'Connell. "The set-piece was good.
"We put a lot of work into the scrum and have had some really good sessions. We're really happy with that part of our game. We put in the hard work and it's paid dividends."