Global enabling of interrupts is done by writing a '1' to the Global Interrupt Enable bit (I) in the CPU Status register (CPU.SREG). To disable interrupts globally, write a '0' to the I bit in CPU.SREG.
The desired interrupt lines must also be enabled in the respective peripheral, by writing to the peripheral's Interrupt Control register ([peripheral].INTCTRL).
Interrupt flags are not automatically cleared after the interrupt is executed. The respective INTFLAGS register descriptions provide information on how to clear specific flags.