This is the 7798th most frequent Russian word.
исилькульского
"Isilkulskogo" relates to something originating from or associated with Isilkul, a town in Russia.
The word 'Исилькульского' is used as a genitive case adjective to describe the author, indicating that they originate from Isilkul.
Я купил книгу Исилькульского автора.
I bought a book by an author from Isilkul.
Here, 'Исилькульского' modifies 'района' (district) to specify the location of the district as related to Isilkul.
Мы посетили музей Исилькульского района.
We visited the museum of the Isilkul district.
In this sentence, 'Исилькульского' describes the folklore, emphasizing that the style or tradition is associated with the culture of Isilkul.