"The cards were introduced to Rugby Union in the 2000/01 season.
The new law stated:
10.6 YELLOW AND RED CARDS
(a) When a player has been cautioned and temporarily suspended in an
International match the referee will show that player a yellow card.
(b) When a player has been sent off in an International match, the referee
will show that player a red card.
(c) For other matches the Match Organiser or Union having jurisdiction over
the match may decide upon the use of yellow and red cards.
The first recipient in a Test match was Ben Clarke, playing for England against Ireland at Lansdowne Road. He stamped on his Bath club mate Simon Geoghegan in the 63rd minute and was shown a yellow card, but played on. Playing on after a yellow card remained law till after the 1999 Rugby World Cup. Then temporary suspension of ten minutes was introduced, signalled by the brandishing of a yellow card.
The first yellow card shown in an international was during the All blacks 1995 tour of France when Irish referee Gordon Black showed it to the All blacks lock Mark Cooksley after he'd punched an opponent in a midweek match at Nancy. The ref later found out that it had yet to be introduced officially but it was shortly afterwards.
The first yellow card shown at a world cup was to Argentina's Roberto Grau (ARG) by referee Paddy O'Brien in the 15th minute of the match against Wales in Cardiff at RWC 1999, whereas, Italian Alessandro Moscardi holds the record for most yellow cards at a world cup. At RWC 1999, Moscardi was sent to the sin bin in matches against England, Tonga and New Zealand." (Source:
http://www.rugbyfootballhistory.com )