There are two main routes to get to France
Landbridge via Britain leaving Ireland from Rosslare, Dublin, Warrenpoint, Belfast, Larne then multiple crossing routes of the English Channel
Direct from Ireland to France leaving from Dublin, Cork, Rosslare to either Roscoff or Cherbourg.The operators are Irish Ferries, Stena and Brittany Ferries. Duration 16/20hours. Two styles of ferry, either the so called ‘cruise ferry’ which have more facilities and tend to be more expensive. Also the economy type with less facilities and in some cases your Motorhome is on an open deck for the crossing. The most cost effective are usually Stena and Irish Ferries. Some deals are available from time to time, with 10% up to maybe 25% discount. Difficult to predict when offers are on.
Each leg can be booked separately so possible to go out from say Rosslare and return via Britain. It may be cheaper to come back via Britain in July but you have to weigh up the driving time and fuel costs. Cheapest English Channel is often the tunnel dependant on length of Motorhome.