Employer holding last salary until work laptop is returned. Can they?

In the U.S. -- absolutely not. If you worked (as a regular employee) you are entitled to every penny of your compensation, with no preconditions.

I don't know European standards directly, but I'd be very surprised if they were any different in regard to such matters.

Anything I can do?

Get a respectable lawyer, pay for a consult. This is a routine matter for them, and they should be able to give you a solid answer instantly. Shouldn't be more than 300-400 EUR (if that) which is a huge bargain, both for the learning experience -- and for innoculating yourself against further insults of this kind.

If you're an employee of the company, then no, they can't withhold wages.

https://www.citizensinformation.ie/en/employment/employment-...

If you're a contractor, then the rules around prompt payment of accounts might be relevant:

https://www.cso.ie/en/aboutus/lgdp/cgt/procurement/promptpay...

They should be sending you an empty box with a label on it to ship their company property back to them. Talk to whoever is responsible for ensuring they receive their equipment. That should not be paid by you but it is on them to eat those costs. Most companies will have that shipped as priority and pay extra, so it shouldn't take more than a few days to get back to them, even if they are overseas. Planes fly all day everyday across the world with packages.

It does not matter the cost of the machine. It is their property. Do not keep their property because that would make you a thief and you would be in the wrong. It is completely normal for a company to retrieve any outstanding property they have as they may have data on there which they own, not you. Make sure you clear out any of your personal files before sending it back but do not do anything that would jepardize your relationship with a reference.

Good luck.

Send an email to your supervisor asking them to approve a reimbursement request for couriering the work laptop to the company after getting an estimate from the courier. Ask them if they are also paying for insurance for the contents of the package in transit (in case it is lost or damaged during transit). If they say no, get it in writing.

Send the package. Include the laptop and all the accessories provided by the company (cables, mouse etc.,)

Send the actual reimbursement request along with the pre-approval including proof-of-delivery from the courier.

Save copies of all the communications, and verify that you were reimbursed in full and paid all outstanding wages.

This should work everywhere around the globe (with reasonable companies). If not, go to a lawyer with the copy of the communications you saved.

> returning the machine internationally will take a while.

I'm pretty sure there's many 24HR couriers to choose from, just make them pay for it

What county and state from a residency and employment perspective? If United States, are you a W2 employee or 1099 contractor?