Irish Ferries had a special to France which ended on Thursday past for outward sailings up to mid June. It ran for about 3 weeks. As it is getting into peak season now they are not likely to have another one in near future. Just need to keep your eyes open. Would be worth giving Nutt Travel in Coleraine a call as they have regular deals.
Land Bridge via Holyhead and Dover or Tunnel is usually much cheaper than direct to France in July and August. Both Stena and Irish Ferries offer Land Bridge fares.