The only Irda Iv'e ever used was a Belkin SmartEYE and it used 2 photo devices... so it was full duplex. I see some people define "half duplex" as being able to transmit in two directions at the same time. I do not think this is the proper distinction for this case. The programming protocol only requires an acknowledgement at the end of a sent packet... so it does not require "full duplex" as defined. It only requires that it not see its own echo.

Is there some kind of echo suppressing circuit that would prevent the acknowledge packet from being seen ?

Have you tried it ?