This comes under non-essential travel.
Ireland has a green list of countries for essential travel. The UK is not currently on Ireland's Green List of "safe" countries. Therefore here is what to expect: -
"What to do on entering Ireland from abroad:
The Irish Authorities advise anyone coming into Ireland, apart from Northern Ireland and individuals arriving in Ireland from locations with a security rating of ‘normal precautions’ (“green”), to restrict their movements for 14 days, and this includes citizens and residents returning to Ireland. Restricting your movements means staying indoors in one location and avoiding contact with other people and social situations as much as possible. To ensure that this is being observed all passengers arriving to Ireland from overseas are obliged to complete a mandatory Public Health Passenger Locator Form and to submit it to the relevant authority at their port of entry. For further details please see the Irish Government Advice Page."
So your exH and your DC cannot just rock up for a three day trip. They would need to arrive 2 weeks in advance of the Mass and self-isolate for 14 days. Also, the Mass is unlikely to be a private one just for family, there will be other worshippers in the congregation, many of them likely to be in the older, at risk category. It would be unethical and criminal not to observe the correct precautions. The numbers allowed into the Church will be limited depending on the size of the building so your exH and DC may be denied entry to the Church.
The Anniversary Mass is normally an important event for the family. However, the pandemic has hit Ireland hard, so many families have been forced to have small, private funerals, no comforting hugs, no flowers, no viewing of the body, no touching of the coffin, often no funeral service, just straight to the grave in recent months. For a family to expect other family members to travel to attend an anniversary Mass at the current time is sheer bloody minded idiocy and an insult to all of us who have been unable to attend the funerals of those we love who have died whether from Covid or other causes in recent months. The message is still Stay Home, Stay Safe.
This pandemic will pass in time and there will be plenty of Anniversary Masses to attend in future years.
Perhaps the Church where the Mass will be celebrated has a livestream of its services so you could watch from home? Search the parish website to see.