A group of Russian-Koreans formed a Korean section of the Irkutsk Communist Party. This group was headed by Russian-Korean Nam Man-chun (남만춘), also known as Pavel Nikiforovich Nam. This section would eventually be renamed as Han-in Kongsang-tang (All-Russian Korean Communist Party), which would then later on be known as the Irkutsk Group. The dates of the formation of this group are sketchy.
The Korean Communist Movement 1918-1948 by Dae-sook Suh dates it as January 22, 1918 (page 10). Communism in Korea by Robert A. Scalapino and Chong-sik Lee date this as January 22, 1919 (page 9). Other sources (russian wikipedia) say 1920.
The Scalapino/Lee book however cites the April 7, 1920 issue of Chokki (“Red Flag”), which was the weekly newspaper published by the Irkutsk Communist Party, Korean Section. As I cannot read Russian, nor have access to primary sources, I can only repeat what I have read in my books.