I don't understand UEFA's logic here.
Both teams were happy for the game to go ahead in Barcelona, which has been used before. This would have accomodated more travelling fans.
Barcelona was the best choice for an away game this campaign so I imagine thousands would have travelled.
Russia and Macedonia had to play in Andorra. UEFA said Ireland would have an unfair advantage by playing in Barcelona. Seems logical to me, but a real pity they couldnt have moved it based on the huge support that would travel to Barcelona.