The cheapest month to fly to Carcassonne from Ireland is in November. Flight prices, which tend to average around 46€, are often lower during this time because this is during the off-season. If you don’t want to visit Carcassonne in November, then you should wait until October to fly from Ireland, when ticket prices are also cheap, with an average price of 54€.
You are more likely to see cheaper flights to Carcassonne from Ireland when booking 60 days ahead of your departure date. In general, you can expect to find flights to Carcassonne for around 119€ when searching 60 days out. Don’t hesitate to check prices now though, because momondo users recently found tickets for as low as 92€.