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As long as you have the right overseas roaming tariff on your mobile plan your sim will work in all of the places you'll visit.
To add it maybe beneficial to speak with your mobile network provider to ensure no abroad bars are in place before you fly.
Also as a back up if you can make sure your phone is unlocked to all network's and it supports the bands and frequencies the Networks use in the UK / Ireland to be able to use a local Sim Card if needed.
Current Phone > Samsung Note 9 _512Gb > Model: SM-N960 Hybrid-Duos.
Samsung One Ui / Pie.
Samsung Gear s3 Frontier Watch.
Thanks for getting in touch @Marvie, I hope you have a really lovely time visiting Ireland and Scotland. As previously advised, the best thing you can do is check with your network provider what their roaming charges would be.
As @63johnw mentioned, if your phone is unlocked to use on any network; it could be easier to get a local Pay as you go SIM to save money.
If you have any other questions, don't hesitate to get back in touch 😊