The Leicester Tigers No9 came off the bench to replace Munsters Tomas OLeary in the 56th minute with the Irish 17-9 in front and played his part as they ran in four more tries.
Murphy touched down in the 70th minute after some powerful work from the Irish forwards.
Tigers colleague Dave Young was at tighthead prop in Scotlands four-try victory over Argentina Pumas in Kingston which booked a place in the tournament final.
The Scots followed Saturdays victory over Canada with a 27-24 verdict against Argentina for whom fly-half Ignacio Mieres kicked two conversions.
Ireland A face England Saxons on Saturday, with a place in the final on the line.