There are two resistors for each ir-led. R1 and R2
for the led on J3 and R3 and R4 for the led on J4.
It can also be seen on the schematics and layout
which are attached.

there is a resistor that controls the power of the
ir-led. andreas tag will tell you more about this.
Changing the voltage of the power supply does
not help. There is a 5V voltage regulator on board.

If the power of the tag is still not sufficient,
you can also connect the tag to a booster.
If you need, I will send the schematics to you.

In general, you do NOT have to have line of sight.
Due to the low bit rate, there is no 'multi-path-
fading'. The effect is comparable to a tv remote
control, which allows also to work out of sight,
if there are walls to reflect the ir-light.

The datasheets of the leds and the FET are attached
on this mail. The maximum continous forward current
of the led is 100mA. The data bursts have a duty cycle
of 0.5 and a pulse time of 26us, so 200mA are possible.
The maximum continous current of the FET is 115mA and
a maximum pulsed current of up to 800mA.