I'm a Jewish Cogan, living in California. I always explain my Irish-sounding name as the Rumanian spelling of the Russian pronunciation of the Hebrew name that, in English, we spell "Cohen".
I'm named for my paternal grandfather, so my Hebrew/Yiddish first name is Sholom (or Shalom or Sholem), my Russian/Ukrainian name is Shukah, and my Rumanian/Romany name is Selim. When he came to America, Grandfather adopted the English-language first name of Solomon, but my parents didn't want their kid to be called "Solly", so I'm Sherwin.
There are a substantial number of Jewish Kogans, Kogens, Kagans, Kaganoviches, etc., but not that many who spell it with a "C". Most of the names probably come from "Cohen", although Arthur Koestler (I think it was) asserted that it came from the Central Asian "Kagan". All this variety is complicated by the multiple languages and alphabets involved. The name gets not only translated but transliterated, from the Hebrew alphabet to the Russian Cyrilic alphabet to the English version of the Latin alphabet. (That last was usually done by immigration officials more familiar with the Irish "Cogan" than the Russian one.) In my case, it was further affected by transition through the Rumanian version of the Latin alphabet.
My late father, Isaac Cogan, was born in what was then called Bessarabia, a province of the Russian Empire bordering on Rumania. Since the Russian alphabet doesn't use a hard "C" (Saint Cyril is really "Kyril") the name was then spelled with the Cyrilic "K".
In the aftermath of World War One, and the Russian revolutions and civil war, the Kingdom of Rumania annexed it as "Moldava". (Russia took it back in 1939 and named it the Moldavian Soviet Socialist Republic; it is now a separate nation, Moldova.) As a result, the family emigrated under a Rumanian passport. The Rumanian version of the Latin alphabet uses a hard "C", not a "K". Presto change-o: Cogan!
The family settled in Milwaukee, Wisconsin. There were two boys and two girls. Each of the boys got married and had two sons, and all of us moved away. As a result, there are now Cogan-surnamed descendants of Shukah in the New York/New England area, in Texas, in the San Francisco Bay area, and here in Southern California, with our own children and now grandchildren moving even farther afield.