Any vehicle insurance policy, from any EU country, provides the minimum level of insurance cover (which is usually 'third party only') required in any EU country where the vehicle is driven. So your vehicle is 'legal', in terms of insurance cover, in France, Germany, Latvia, Spain, Estonia and everywhere else in the EU, including Ireland.
However additional elements of insurance cover (i.e. beyond the minimum required by law) do not normally apply when the vehicle is taken outside of the country where the policy is based. So, if you've got comprehensive cover (and you want it to cover you while in Ireland) you should seek to your insurer to get an extension of that cover.
This map shows your choice of ferry routes:
http://www.eurodrive....mages/misc/routes.gif
Given the rough crossings which can be experienced at this time of the year, I'd suggest using the shortest crossing, from Holyhead. The fastest service (to Dun Laoghaire) is considerably more expensive for a van than the (slightly slower) vessels to Dublin:
http://www.stenaline....s/pricing-to-ireland/
Chris