It means the first son.Fun fact: MLB player Ichiro Suzuki is given this name even though he is the second son.
― Anonymous User 1/28/2021
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Ichirou, used as the kanji "一郎" is currently 37th place and "一朗" is currently 329th place in Japan in terms of kanji and Ichirou used as "いちろう" is 197th place in terms of sound. (source: http://5go.biz/sei/cgi/ninki1.htm).
Fun fact:
MLB player Ichiro Suzuki is given this name even though he is the second son.