Yep as already stated get a moving firm in.
Get quotes from local companys.
They will vist you and assess your stuff and quote a volume in cubic feet.
For rough ideas, a standard 20 foot box 7.5ton lorry is around 900 cubic feet, the big double decker bus size Pantechnicons removal lorries are around 2,300 cubic feet of volume.
if you get a quote in volume, you can use that over the phone for a blind quote with Irish based movers, just remember to try and ditch some stuff, and don't add anything/hide anything as you might run out of space.......... Did it for 4 years, loads of people trying it on....... get a quote then have new furniture delivered a week before moving, then have a paddy because they are 500 cubic feet over volume and it won't fit the truck.
Sending over 1 large lorry load might be marginaly cheaper with a moving company. Should be less hassle. Cost that against rental fee for self move, time taken, and double the fuel and ferry costs.
Be ruthless, anything you don't think you need, get rid of it. We moved people who had stuff still boxed from a move several years earlier, just clear all the things out. This will help speed up the actual day of moving out, and speed up getting everything in the new home.
Start hoarding cardboard boxes, box all the things :)