In the Italian forum you said you're using a [glow]DL[/glow]271. I coun't find this datasheet.
But if you make a mistake, and the IR is a [glow]LD[/glow]271, the datasheet is here:http://www.ortodoxism.ro/datasheets/siemens/LD271HL.pdf
It says that the forward current is 130mA. and forward voltage is 1.3v.
So, you cannot use arduino pins to connect it directly (Arduino pins max. current is 40mA per pin). If it was possible, the resistor value was supposed to be 28ohms (27 in commercial values).
So you need to use the multiple leds circuit as an example, with one IR led and one resistor with 18ohms. This resistor will dissipate .312 W, so you'll need a 1/2 watt resistor.
Be careful if you want to connect many IR led, because of the Arduino regulator current limit. If you need to exceed it (I'm not sure but diecimila is 800mA, serial 1A), you can change the multiple leds circuit to use 9v. (with 47ohm resistor in series with led).