Had a wee look at the following:
Seatwave (used them for concert tickets - they are affilliated I think to ticketmaster and you can choose the block)
http://www.seatwave.com/republic-of-ireland-tickets/aviva-stadium-tickets/13-june-2015/perf/980933
donedeal.ie (cheapest but dont know much about them)
viagogo.ie (used viagogo before and always good - need to kid on you live in Ireland or get someone from Ireland to buy it)
needaticket.ie (seems good but never used it)