I'm not very sure regarding the Republic side of things, but if by some luck your first destination is the UK (Northern Ireland) then I can recommend getting a PAYG (Pay As You Go) Three sim card. You should be able to just find it in shops. Once you have it, you can either top up online (I believe you should be able to access three.co.uk for that even without any credit on your sim), or by visiting Three's branch/shops that have green Top-up logo.
At the time of writing this, you can convert your money into the following addons:
- £10 - 2GB
- £15 - 5GB
- £20 - 12GB
- £27.50 - 36GB
- £35 - Unlimited in the UK (it's actually 2TB).
All of the above include unlimited local phone calls/texts and last 30 days.
Now for the major benefit of the SIM, you can use up to 15GB of data from the available allowance while traveling abroad in the EU, i.e. to the Republic of Ireland! (can use up to 12GB in the USA even!)
Three even confirmed in case of Brexit their free roaming in 71 countries policy will not change.
Regarding device compatibility, it is true that networks are different in Americas and elsewhere.
Three UK uses the following frequencies:
- Band 20 | 800MHz for 4G
- Band 3 | 1800MHz for 4G
- Band 1 | 2100MHz for 3G
Looking at gsmarena.com, Samsung Galaxy S7 Edge seems to support all the above bands/frequencies, same goes for Samsung Galaxy S9 Plus . So unless those phones sold in Canada have different specs than what gsmarena mentions, you should be fine to use Three sims in the UK (Northern Ireland)!
For the Republic specifically, Three also exists there and from browsing at their website, I believe you can get a prepay SIM with unlimited data for €20 (8GB of which you can use in the EU, i.e. Northern Ireland - not sure about the Brexit situation this way around).
Frequencies used by Three IE (couldn't find the bands though):
- 800MHz & 1800MHz for 4G
- 2100MHz & 900MHz for 3G
- 900MHz & 1800MHz for 2G
Enjoy the holiday!