Ööö . IRDA is serial comms over infrared (RX/TX), afaik N900 is/was only capable of transmitting IR (like with Irreco app)

Hmm... Using just the UART to send IR 'bits' with carrier frequency could be possible. The carrier is about 30...60 kHz, say 40kHz. If this is set to uart bitrate.
The actual datarate is way much lower, say something like 1200 bps. So we need send about 4 bytes of 0xAA for one high bit (and 4 bytes of 0x00 for one low bit (hoping that the STOP bit doesn't cause problems)

Doing it like this would free the phone cpu from low-level timing and lets the uart to do it. just feed the data to fifo over i2c.

Could be doable. Just add buffer and IR led there.
For IRDA something like TFBS4711 could be suitable.