The pair were welcomed by the club chairman George Comitis at the Athlone Stadium at the weekend.
Harpur played his football in the United Kingdom with Scottish Premier League side Kilmarnock. He has also played for Leeds United and Millwall.
The 26-year-old has also had stints with Manning Rangers and Maritzburg United and represented the South African Under-23 team. His return to South Africa is due to his ambition to stake a claim for a place in the national team at senior level ahead of the 2010 World Cup.
Carbonnier, a young Frenchman who played for Ajaccio in the French Ligue 2, is 22-years-old.
Standing 1.8m tall, he played as a striker for Montpellier in Ligue 1 from 2002 to 2005 before he joined Ajaccio in the 2007-08 season.