Dear,
Yoon is the 5th or 6th most popular family name in Korea.
Most Korean first name consists of two syllables like mine;
Hyoung-Jin, for example. Usually only the first syllable of the first name is used since it is very difficult for foreigners to pronounce the entire name.
Roman Catholic has two hundred years of history in Korea. At the end of late eighteen hundreds, the believers were persecuted by the government because of their religion.
H Yoon might have been one of the refugee fled to other countries, where he married.