Ireland lost their opening Super Eight match against England but only by 48 runs and Birrell insists they can be competitive against the Proteas at the Providence Stadium.
“Our confidence has certainly risen since we arrived in the Caribbean,” said Birrell. “We have a game plan and I believe that once we stick to it we will be competitive.
“Members of the other teams have being very supportive but we feel that some press reports have not given us the credit that is due.
“However, a good performance against South Africa can certainly enhance what we have done.”
Ireland almost caused an upset in a warm-up match against Mickey Arthur’s side before the tournament got underway.
Seam duo Trent Johnston and Dave Langford-Smith helped reduce South Africa to 91 for eight in Trinidad but they recovered to post 190 and Ireland were bowled out for 157 in reply.
Ireland skipper Johnston said: “We played well in the warm-up against South Africa and then during our win over Pakistan so if we can repeat those two performances we can achieve the best results.
“But we need to lift our performance by at least 25% in the remaining matches.”
Arthur insisted that his South Africa players will not be taking Ireland lightly.
“Ireland may be a team of amateurs but they will always be tough because they play for one another with tremendous passion.”