What kind of support does a 15 inch overhang on an island require?

[NYC_SKP]

1. 15 inches is subtantial.

A lot depends on the material used for the countertop, if it's tile or granite then you can't have any flex.

For a 15 inch overhang I'd want to see brackets that extend out 11 or 12 inches under that 15 inch overhand, and a 3:2 proportion, or higher for the vertical leg, which would be a 16 to 18 inch vertical leg, or greater.

These need to be spaced according to the materials and dimensions at hand; two brackets might do for a 36 to 48 inch surface, I'd add one bracket for every 16-24 inch increment beyond 48 inches.

Hope this helps.