CIR output is PWM type so there is no brightness control, just only on ( maximum ) and off
but between "always on" and off state can be a lot of average brightness values.

from datasheet :
• Transmit mode only (receive mode is not supported)
• Free data format (supports any remote-control private standards)
• Selectable bit rate
• Configurable carrier frequency
• 1/2, 5/12, 1/3, or 1/4 carrier duty cycle