This question already has an answer here:
I am an Irish citizen, along with my whole family.
My in-laws are non-EU citizens. They travelled to the UK on a visit visa while they applied for an Irish visit visa in their home country. During their stay in the UK, their Irish visa was approved. After facilitation through the Department of Justice visa section, they got their visa at the Irish Embassy London.
Two days later, they came to Ireland via cheap budget flight, landing at Belfast airport because they wanted to see Belfast, as well. Then, we travelled via road in my Irish registered car to Dublin. They stayed two weeks and flew back to the UK via Ryan.
In this whole process their passports were never stamped. In fact, apart from their airline tickets, there is no evidence they had travelled to Ireland. They applied for another Irish visit visa. They wrote in their visa application that they did travel to Ireland, but the visa section is asking for proof of the stamp for that visit. How can we explain that there is no border control so no stamp was put in?
Was their entry illegal even though they had valid visas for both the UK and Ireland? Should we have done something differently?