Tiebreakers aren't decided on goal difference in this, it's head-to-head records that matter. Ireland have won 1-0 in Armenia and in terms of the head-to-head have beaten Slovakia on away goals. If we finish level on points with either (unless Armenia beat us 2-0 or something) we will be ahead of them, regardless of goal difference.
The real question is what we want to happen in the Slovakia-Russia game. A Russian win practically guarantees us a play-off place (assuming we beat Andorra and get at least a draw with Armenia), but a Slovakian win means we can win the group by beating Armenia. However, if we draw with Armenia in that instance (which is very possible on recent performances) we would miss out altogether.
I'll be hoping for a Slovakian win, because for me we're more likely to beat Armenia in Dublin than to beat Turkey or Sweden over two legs.