You cannot evolve Eevee into Umbreon or Espeon in FR/LG. Per Bulbapedia:
In Pokémon FireRed and LeafGreen, however, the time function is completely absent, allowing it to be more like the Generation I games. This also makes it impossible for Eevee to evolve into either Espeon or Umbreon in these games.
In R/S/E there is a time system, so you can trade your Eevee to those games and evolve it there. Raise Eevee's happiness and level it up at the appropriate time to get Umbreon or Espeon. Then, trade it back. Remember that you need the National Dex to do this in FR/LG.
You can also trade one from Pokemon Colosseum or XD: Gale of Darkness.
