Teammates at Leicester Tigers and partners in Timo Italian restaurant in Market Harborough, Murphy and Castrogiovanni will put their friendship to one side on Sunday for their winner-takes-all clash in Dunedin.
And Murphy joked he had the ideal way to distract Castrogiovanni at scrum time.
"I'm big into my scrummaging, so I'll definitely be telling my forwards about how to scrummage him!" joked Murphy.
"I'm thinking of leaving some baskets of chips in the corners of the changing room for him. That should do it.
"The amount he eats, it should distract him quite nicely.
“He's a good mate of mine, he's a fantastic prop and he's a different character off the field to what he is on it. He's certainly one of Italy's key players.
"He's been texting me. The closer the game gets, I'm receiving more and more text messages from him.
"He swears a lot at me in Italian and he's trying to teach me all the bad words. I'm not going to repeat them."
Ireland top Pool C going in to the final round of games. Australia, who play Russia, are second, level on points with Italy. Ireland will finish top of the pool with victory but a win for Italy will see them qualify for the quarter-finals and leave the Irish sweating on Australia’s result.