you should certainly look at the plans which should be available at your local council offices, assuming your neighbour hasn't done the decent thing and sropped a copy around to you
Depending on the circumstances, even if planning is approved, it may be approved with changes which will reduce the impact on yourselves. However, before objecting, and if you get on with your neighbour, a quick chat about things like windows overlooking your garden may get issues resolved
If your objection is appealed to An Bord Pleannala by yourself(if you were unsuccesful) or your neigbour(if your objection was upheld and you PP was refused) then you are into a bit of a lottery. In my own circumstances, I objected to a neighbours PP, the council turned him down, he reapplied, got refused again, appealed to An Bord Pleannla who's assessor recomended that it be refused for all the reasons the council refused him, yet some senior person on ABP granted the permission. We ended up having no choice but to sell as a result.