Assuming that the fee was £7m...
I don't really understand why people think that £7m is a particularly large amount of money for a 23 year old who is one of Ireland's better players and has shown that he is good enough to play in this league. Especially a left footed player, they are in short supply.
I personally think it is well worth paying a premium for players who have shown their mettle in this league, over imported players who are always a gamble and hit-and-miss.
The alternative was probably signing Ross McCormack for a similar kind of fee and playing him out wide, and he'd probably bring a few more goals, but the difference is that he's 28, completely untested at this level, can't play wing back, can't play left back, and won't retain his value because he should currently be at his peak or very close to it. He can play up front, but he doesn't improve on what we have in that position.
Or taking a punt on an import, that's always a gamble.
People don't seem to have noticed that we've got a relatively old squad, plenty of players 28/29/30/31. So I'm happy that we gone for a player who hopefully has his best years ahead of him and may still be here in 5 years time.