It's a commercial lease, so the tenant has a right to a new lease once they are there for over 5 years. (Unless they have signed a renunciation document)
Contact your solicitor and a property agent.
Get a new rent valuation from the agent (there will be a fee of €1,000-€1500) which will be based on similar properties. Let the tenant know you are doing this. They can employ another agent to negotiate on their behalf.
Tell the tenant you are offering a new lease at the new rental level.
The tenant then can accept or not. The solicitor will guide you.