Kirwa made her international debut for Kenya at the age of fifteen at the inaugural 1999 World Youth Championships in Athletics. She finished fifth over 1500 metres at the competition, while her compatriot Sylvia Kibet took silver. She returned to the distance at the 2001 World Youth Championships in Athletics but did not make the final on that occasion. She did not compete at major events over the next few years and tried out the steeplechase instead. She was runner-up at the 2004 North Rift Championships in the 2000 m steeplechase, then came third in the 3000 m version at the Kenyan Athletics Championships later that year.