After course completion and expenses, can I do any business or is it only job-related in UK/Ireland for a PhD?

Whether you can start a business after your PhD in the UK or Ireland comes down entirely to which specific post study visa or status you move into, since your student visa itself never allows self employment. The UK gives you real freedom to launch a company once you are on the Graduate Route, while Ireland's equivalent status keeps you restricted to standard employment. Your long term entrepreneurial options also differ between the two countries.

Business Rights By Country And Status

Your Long Term Entrepreneurship Options

• In the UK, you can switch from your Graduate Route to an Innovator Founder Visa if your business idea gets endorsed by an approved body as innovative, viable and scalable.
• In Ireland, non-EEA nationals can look into the Start-up Entrepreneur Programme, which requires a minimum funding threshold of around 50,000 euros and a job creating business proposal.
• Alternatively in Ireland, transitioning to a Stamp 4 through enough time on Stamp 1 or a Critical Skills track grants unrestricted rights to work and start a business.
• During your actual PhD, both countries strictly prohibit self employment, so any business plans have to wait until you reach the right post study status.

My Advice

Decide early which country's post study status actually fits your entrepreneurial timeline, since the UK's Graduate Route opens business rights immediately while Ireland's Stamp 1G forces you toward standard employment first. If Ireland is your only option, start researching the Start-up Entrepreneur Programme's funding threshold well ahead of time, so the 50,000 euro requirement does not catch you unprepared.