Using Arduino Mega to wirelessly transmit IR signals
You need to define the ACU, carrier frequency of IR (30kHz is most common) then auto-learn the data format from your existing remote then duplicate it with an AND GATE software or hardware with NRZ synchronous data and clock. Drive current 100mA may be plenty 300mA for long-range or get narrow 10 deg. beamwidth LEDs for even more range to bounce off walls.  

If 1.4V~1.5V from 3V @ 100mA then you want 1.6V~1.5 drop or 16~15 Ohms in series from a low Ron FET switch or direct drive from a 22 oHm CMOS port in the 3.3V Mega. 

For 300mA from two AA's @ 3V the IR 5mm LED Vf rises to about 1.9V typ.
(Vbat{max,min}-Vf{LED@ If})/Rs= If of LED
The incremental resistance of the IR323 LED is about 5 Ohms and the CMOS ATMega drivers are about 25 Ohms.

Driving the LED is easy once you specify the supply voltage like 2.5 to 3V (2x Alk.bat or 3.3 2% or 3 to 3.7V LiIon.  The LED can handle more than 100mA continuous depending on the duty cycle and thus average power.  The 1A and 1.5A max ratings are at 10us. But you will be using something like 50% max duty cycle of 30kHz approx.  The LED can be modelled with a linear regression so a series R can limit the current. The formula depends on the datasheet curves or you can use a linear Op Amp current sense limit with FET to accurately drive the LED if you want with a CMOS OA, logic level FET and &lt;=1 Ohm current sensor.

http://www.vishay.com/docs/82489/tsop322.pdf
This will show you how the common carrier modulated receiver is expected to work in your ACU.