Stepping down I2C data voltage
For the SDA and SCL lines, thus the I2C interface, a level shifting is necessary. Without the level shifting the communication will not be possible.

The simplest way to accomplish this is via an I2C level shifter (for example see in the website of NXP or TI). The important here is to have a bidirectional buffer/level translator. In practice the I2C level shifters are indeed bidirectional, but I just mention it just in case you use some other buffer not dedicated for I2C.

Now, regarding the IRQ pin. As far as I understand this is output, correct? And I also see that it is an open drain pin. That means that you have to connect an external pull-up resistor to the power supply of the receiver, aka the PIC microcontroller. That will do the job and no other special action is necessary.

The open drain configuration in practice means that the MPR121 IC can only pull the pin to GND. So, the LOW state of the IRQ signal will be understood without problem from the PIC. For the HIGH state, the MPR121 IC will not actively drive the pin, so the pull-up resistor alone will pull the state to the power supply of the PIC and in this way the PIC will be able to understand the HIGH level as well.