I am going to assume you are remote controlling a TV or something? Using the IR remote library.
You need to look at the datasheet for your IR emitter. One I used recently: http://cpc.farnell.com/osram/ld271/ir-emitter-5mm-950nm/dp/SC07713?Ntt=SC07713
has a forward current of 100mA at a forward voltage of 1.9V. Which is too much for the meagre 40mA of an Arduino output pin. So you would need to use a transistor, and resistor, in an arrangement like this:http://www.robotroom.com/Infrared555.html
See the explanation on the link above for deciding on your led current limiting resistor value.