Depends on how/with whom you fly and what are the carry-on rules
European usual 8 kilos --> camera has to go as checked luggage
European unusual 20+ Kilo --> get a photo backpack or trolley and bring it in the plane
If you need to send the camera as checked luggage then i suggest a Pelicase for the camera rig, maybe together with delicate Rebreather parts
If you have more than 15 kilo carry on then housing, strobes, hard disks, chargers, laptop in the photo backpack/trolley, alrms, cables, spare parts in one of the luggages
I can store my Poseidon SE7EN in a 1610 Pelicase, wing and backplate, fins, wetsuit, mask go in the same big suitcase (will be more than 23 kilos/50 lbs)
Second suitcase has cloth and other stuff (maybe drysuit) and if carry-on is not possible then the second pelicase for the camera rig
1 Messenger bag (if allowed as 2nd carry-on) for the laptop, noise cancelling headphones, newspaper
The trick is to check first the luggage rules and start to make some math, probably cheaper to pay additional checked suitcase than overweight carry-on
Probably not cheaper, but not much more expensive to fly business with higher luggage/carry-on rules and much more comfortable
I would not recommend a Pelicase carry-on as the heavy trolley will make limited carry-on weight even worse, a photo backpack is lighter.
Guerilla warefare:
Buy trousers with zipped cargo pockets and a photographers/fishermans vest with a lot of pockets. Then store all heavy lenses, the camera, tickets, passport, etc in it to lower carry-on weight
You can also put a cord on your housing and hang it around your neck as a camera is allowed as carry on.
Instead of a expensive photo backpack/trolley you can get also a normal trolley and a roll of bubble wrap to wrap everything well.
However, get prepared to pull everything out of the carry-on during the security check, agents there hate trolleys full with electronic stuff ...
US Ailines usually have no weight limit for carry-on, so a photo trolley will work fine, but if you have a inner-european flight to Ireland then your carry-on will be most likely limited to 8 kilo. Check that in advance!
Chris