It depends where you want to enter mainland Europe from Ireland. For driving through France, buy 'All the Aires' from Vicarious Books. (Order on line). There are two books, one for the north and one for the south, but you get a discount if you buy them together. They are far from all the aires but there are loads......plenty enough for you to find overnight stops free, or for a few Euros. Almost all also offer a facility for dumping black and grey waste, and for refilling with water.
We do the Tunnel, (never a problem around Calais), France, Belgium, Luxembourg (cheapest fuel!), Eastern France again, Switzerland and Italy. Friends with a house on Andros and an apartment in Athens.... and a camper-van to get to them both... do the same route. It is a well travelled route for them and us!
In Italy most of the motorway service areas allow you to stay without cost overnight. They also seem to have servicing
facilities. If you are just passing through to get to a ferry they are adequate for an overnight stop. Switzerland is more of a problem. You will also need a vignette to drive there. - If you are over 3.5T you can buy one for 10 days, (any 10 days in a year.......so you can use it travelling through on your outward and return trips). Surprisingly it is cheaper than the vignette.
If you are not looking to have a holiday in Italy, but just want to get to a ferry, then Ancona to Patras is the best route. (About a 24 hour crossing). Bari to Patras is probably the next best. (About 17 hours). Bari is cheaper, but by the time you have paid the fuel and autostrada tolls to get to Bari there is very little difference in price. ANEK/Superfast and Minoan do both the route from Ancona and offer 'Camping on Board ' from the 1st April to the 31st October. With ANEK/Superfast you can stay in your van and get an ehu. With Minoan they give you a cabin but you cannot return to your van, therefore have to buy your meals etc on board. There are showers and loos on the Camping deck on the ANEK ships. It is VERY popular, so you would need to book up in advance. I don't think Minoan do the Bari crossing. We have done it with ANEK/Superfast and the ship's are smaller than from Ancona......but fine. It tends to get less booked up than Ancona outside of July/August/September, but you would still be sensible to book ahead.
Warwick is correct. - You get a discount on the domestic leg (PIraeus to Hania or Iraklio) if you book it with the same shipping line. If you book with Viamare in London (on line or by 'phone) they can sometimes find you an 'open return' for the same price as a fixed one. They are also usually the cheapest agency to book through. - Even cheaper than booking directly with the ferry companies. If you are a member of the AA, Minoan give you a 33% discount. If you are a member of ADAC you can get one with Minoan and ANEK.
All the information above is not necessarily true for 2017, so you will need to check!
If you want any more details then pm me.