not for direct sunlight but just to obtain a UV Index to within 1 index accuracy.
As I'm sure you've noticed from the SI1145 data sheet, it does not measure UV, but estimates the UV Index from the visible and IR. That estimate is reasonably accurate only on a cloudless, sunny day.
In any case, you don't need to worry about the dome passing UV -- it just has to be transparent to visible and IR, from 400 to 1100 nm.