Loose head prop Parr heads to his new club in the summer on the back of stints at Leicester Tigers, Sale Sharks and Saracens, while second row Rouse has played over 160 games.
Director of rugby Brian Smith told the club's official website: "We are delighted to have these two players on board for the next few seasons. Matt will bring massive strength, work rate and experience in a key position which will be crucial for us going forward.
"Nic also has a great work rate and is a very physical player. Glenn Delaney has worked with both Nic and Matt and is very happy that both will be joining us next season."
Parr spoke of his move by saying: "I'm looking forward to joining London Irish next season. They are an ambitious club with some great players. Until then I will be concentrating on Nottingham and the club's promotion push."
Rouse added: "It will be an exciting challenge when I join London Irish in the summer. They have really showed in the last few months that they are a top side and I am sure we can push on next season. There is still work to be done at Nottingham and that will have my full attention until the end of this season."
Both players have penned two-year contracts.