In addition to globally enabling interrupts, each device's particular interrupt needs to be enabled separately if interrupts for this device are desired. While some devices maintain their interrupt enable bit inside the device's register set, external and timer interrupts have system-wide configuration registers.

Example:

// Enable timer 1 overflow interrupts.
timer_enable_int(_BV(TOIE1));

// Do some work...

// Disable all timer interrupts.
timer_enable_int(0);