This is because of something called the Common Travel Area (CTA) which is an arrangement between the two countries (……as well as the Crown Dependencies of Jersey, Guernsey and the Isle of Man). As a consequence of the CTA, British and Irish citizens are allowed to reside and work freely in either jurisdiction, as well as access social welfare benefits and health care. They can even vote in some elections!
After Britain left the EU in 2020, it became more complicated for UK companies to trade with Europe and setting up in Ireland became increasingly appealing. The country, already popular with British businesses for its favourable tax system, common language and time zone, became the biggest beneficiary of Brexit related relocations.
Because the CTA exists, businesses in the UK can freely send their employees to work in Ireland. Whether that’s to complete a project, establish a new market or to do the groundwork in establishing a formal entity. However, UK citizens working in Ireland will be subject to the mandatory employment laws of Ireland so employers need to understand the regulations. This is true even if their employment contract states that the terms of employment are governed by UK law.
Similarly, anyone residing and working in Ireland should be paid through an Irish payroll with payroll deductions (PRSI and USDC), totalling a little over 11%, going to the Irish Tax and Customs department. They may even need to set up a Personal Retirement Savings Account (PRSA) for each employee.