Actually 90k , but fair point.100,000 affected people between February and April.
If you received a letter from the HSE about the cyber-attack
We are writing to people whose personal information was illegally accessed and copied during the cyber attack on the HSE. Read advice if you have received a letter from us about the cyber attack.
Though I don't have huge confidence that they either got everyone, or that this data won't surface at some point in the future.
In any event to answer the question, its worth the cost of a stamp to write to HSE.