Interrupts or Inputs ?
74148 is 8 lines to 3, the guy need something like 16 to 4 but nevertheless that will not react like one expect from interrupts, you know:
1) halt normal program
2) jump to Interrupt Service Routine
3) with a RETI resume normal program
So jagjoes if you really need 15 interrupts, and I mean interrupts, 89C51 is not that part you need.
If you need 15 inputs to a µC then consider memory mapped I/O, using I²C I/O chips, part with more I/O...