... globals.h:
#define PIN_RTC_IRQ 2

...Firmware.ino:
#if RADIODUINO_BOARD_REVISION < 2
    // RTC PIN uses internal pullup resistor
    pinMode(PIN_RTC_IRQ, INPUT_PULLUP);
#else
    // REV B and up already have an external pullup
    pinMode(PIN_RTC_IRQ, INPUT);
#endif

... alarms.cpp:
attachInterrupt(digitalPinToInterrupt(PIN_RTC_IRQ), RTCIRQ, FALLING);