Most of the discussion above for InterruptAttach() applies to the InterruptAttachEvent() function, with the obvious exception of the ISR. You don't provide an ISR in this case; the kernel notes that you called InterruptAttachEvent() and handles the interrupt itself. Since you also bound a struct sigevent to the IRQ, the kernel can now dispatch the event. The major advantage is that we avoid a context switch into the ISR and back.
In order to attach and interrupt event, you must have the PROCMGR_AID_INTERRUPTEVENT ability enabled. For more information, see procmgr_ability() in the C Library Reference.
An important point to note is that the kernel automatically performs an InterruptMask() in the interrupt handler. Therefore, it's up to you to perform an InterruptUnmask() when you actually clear the source of the interrupt in your interrupt-handling thread. This is why InterruptMask() and InterruptUnmask() are counting.