She was then seconded to the Ministry of Foreign Affairs as Legal Officer in the same year.
Her first overseas assignment was to the Kenya Permanent Mission to the United Nations in Geneva, Switzerland, as a Legal Officer and 2nd Secretary for four years. Her second overseas assignment was to the Kenya High Commission in London as Legal Officer and 1St Secretary where she served for another four years.
In 1998 she was recalled to the Ministry of Foreign Affairs to head the Legal Department and eventually was transferred to head the America's Department responsible for North and South Americas and the Caribbean.
In June 2000, Amb. Kirui was appointed Kenya's High Commissioner to the United Kingdom and Northern Ireland and accredited Ambassador to Switzerland and the Republic of Ireland. She was in London until June 2003 when she was appointed Permanent Secretary in the Ministry of Gender, Sports, Culture and Social Services. In June 2004, she was appointed Permanent Secretary, Ministry of Labour and Human Resource Development. In December 2006, she was appointed Permanent Secretary Office of the Vice - President and Ministry of Home Affairs. In April, 2008 she was appointed to her present post.
Amb. Kirui is a seasoned diplomat and has considerable experience in the fields of multilateral and bilateral negotiations, public international law, consular and diplomatic relations, international and regional organizations and the United Nations.