Typically the output of an Arduino type board does not have sufficient current to drive an LED so you need a driver circuit using a transistor. Here is a fritzing diagram and a schematic of how to wire up a simple driver circuit. A more complicated multiple LED driver circuit can be found in the documentation for IRLib2.
The base of the transistor is connected to pin 9 of the feather board (or pin 6 of the MKR 1000)using a 470 ohm resistor. The emitter is connected to ground and the LED is connected between the +3.3V and the collector.
The MKR1000 uses pin 6 as its default output pin rather than 9 on the Feather. These defaults can be changed in IRLib2. See it's users manual for details.