The IRQ pin is a normal Push-Pull output.
There is a very good tutorial document available on the internet: nrf24l01_tutorial_0.pdf
In there it says this about the IRQ line:
The remaining two pins are CE and IRQ. CE is used to control data transmission
and reception when in TX and RX modes, respectively. IRQ is the interrupt pin, and is
active-low. There are three internal interrupts that can cause this pin to go low when they
are active. Each of these bits can be masked out such that when the bit’s respective
interrupt becomes active, the status of the IRQ pin is not changed.
It goes on to say:
The SPI pins are all done, so now let’s get to the seventh pin from the top, the
IRQ pin. This guy is basically the interrupt pin to signal your microcontroller that
something interesting has happened. You can set interrupts for any combination of the
following events: data received, data transmitted, and maximum number of transmit
retries reached (this particular one is related to the 24L01’s hardware link-layer, more on
that later). If you’re not using interrupts, this pin isn’t required because you can poll the
24L01’s STATUS register over SPI to see if any interrupt has occurred, but it’s still faster
(in general) to check the status of an IO pin than to send an SPI command and then wait
for the response. NOTE: the IRQ pin is ACTIVE-LOW. It is normally high, but when
an interrupt is asserted, the pin will go low.
Although it doesn't explicitly say it, the wording implies that it is a normal push-pull output, otherwise one would expect the author to make mention of it being open collector and a pull-up resistor being required.
Also, in the datasheet the pin is labelled as a "Digital Output". In the DC characteristics in section 5.6 it lists the output voltages for a "Digital output pin" (table 13). An open collector wouldn't have an output voltage, so it must be push-pull.