Many phones have an option to block/blacklist specified numbers.
For example:
Android: https://www.androidpit.com/how-to-block-calls-on-your-android-phone
iOS: https://support.apple.com/en-ie/HT201229
Of course if it's a serious case of nuisance or harassment calls you should consider contacting the Gardai.
https://hello.donedeal.ie/hc/en-us/articles/201121282-Unwanted-calls
Hope this helps.
