Hey Paul! Thank you very much for your answer.
According to the documentation that my company provides, I should declare Tax when the purchase option is exercised. The taxable amount would be the difference between the Fair Market Value at the time of the purchase and the actual purchase price. Shares are purchased with 15% discount in my case.
Since the plan is an unapproved share scheme, I do think that I need to declare and pay taxes. Here is a description on the revenue website: [broken link removed]
Sadly, the page does not clarify the Income Tax that can needs to be used to calculate.
I would be very grateful for any further advice.