score:6
There are two sides to this.
The first side is frequency bands, the US uses different bads to europe. Top of the line smartphones will usually support a wider range of frequency bands than lower end models. If you take a recent US smartphone to Europe or vice-versa it will probablly work on 3G but has less chance of getting 4G.
The second side is what to do about service, there are two routes.
The first is to get a European roaming deal (you don't want to pay non-deal roaming rates) from your canadian provider. This means you keep your canadian number (which may be a benefit or a drawback) and you can just use your phone immediately you get off the plane. The downside is the prices can be quite high, for example bell canada's "roam better with home data" plan is $12 per day.
The other option would be to get a local pay as you go deal. You can buy the sims in many places and activate/top up with a credit card easilly enough, but afaict you can't do it until you actually get there.
Also complicating this is that you are visiting two different countries. The EU has abolished roaming charges within the EU, but the rules are intended for people who live in one EU country and visit another, carriers can (and sometimes do) terminate people whose roaming usage forms too great a proportion of the total.
And a final wrinkle is brexit, noone knows for sure how it's going to play out and what if any effect it will have on roaming between UK and Irish networks.
Upvote:2
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:
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:
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):
Enjoy the holiday!