I was deliberating which components to use for the two modules. In the end, I decided to use a ATMega328 for the controller/bridge component, and an ATTiny85 for the IR transmitter.

The ATMega328 was chosen for the controller because of the breadth of library code already available. The ATTiny85 was chosen because of its small size and low power consumption, whilst still providing SPI (via the USI) and comprehensive timers (needed for the IR carrier wave).