The story doesn't put a date on the supposed pyramid at Visoko, B&H.
The Illyrians aren't very well known since we don't have anything they wrote, just occasional mentions of them by Greek and Roman authors, plus archaeological evidence. Whether the people in central Bosnia were true Illyrians is uncertain. This may have been the territory of a tribe the Romans called Daesitiates, but whether they were there at the time of the pyramid (if it is a pyramid) is unclear.
There's a good book in English by John Wilkes entitled The Illyrians (Blackwell, 1992). There was a report more recently of discoveries in a cave along the Dalmatian coast (Peljesac peninsula) that may shed light on Illyrian religion.
Yup. I was trying to lump mounds and pyramids found in unexpected places in the same spirit as the below linked book.