I have set up a ltd company and for now I'm using a virtual office registered in Dublin because at the moment I don't need a real office. In fact, I'm working with my customers at their working place.
I have a customer that is from Italy and it is required to go there a couple of days per month.
I would like to know if I can claim back expenses like hotel accommodation and airplane/train tickets.
From who? If you mean from your customer then the default position is: what did you specify in your contract? It would be standard procedure to charge them for at least actual expenses incurred I reckon.