Early history of the chapel are available via government website here https://cco.gov.hk/en/project-stories/sai-kung-peninsula-hakka-people-and-catholicism
Wikipedia lists construction date as 1874 after the original building on this site was destroyed by typhoon in 1867. This is corroborated by the government website link below.
https://en.wikipedia.org/wiki/Historic_churches_of_Sai_Kung_Peninsula
During the Japanese occupation period, and possibly before, this chapel was a base for the HK-Kowloon Brigade of the East River Column.
Details are given on a sign erected next to the chapel in 2025 along with this government website.
https://www.80avictory.gov.hk/en/site-content.html?id=12
The presence of this post seemed to be a direct reason for the BAAG post being set up next door.