I think the only way to find out the 18th century spelling of Schiruska/Chyruska is to take a look into the parish book of Gradlitz/Choustnikovo Hradiste for this period. The parish books are kept in the Archive in Zamrsk. It was planned to publish them via Internet in 2012. Unfortunately this was not possible. Maybe they will be published later this year or next year. As I am personally interested in the Friebel family I´ll try to keep you informed in this subjekt. Nevertheless I think it is possible that the 20th/21th century spelling in german of the name is Schirutschke. It is the only name that I found in kontext with the region that has real similarity to the searched name.
Kind regards, rud_olf