You didn't give us very much information to go on. For instance, you didn't tell us the crucially important information of what carrier you plan to use this phone on. Every carrier uses different frequency bands. AFAIK, the S5 model normally sold in Ireland is the 900F. You are asking about a model that is intended for use in another country and as a result it may only be partially compatible with the frequencies used by your carrier. You can check the specification for the different models to confirm whether they are compatible.
According to Samsung, there is no "S5 Prime". The S5 Prime was rumoured all along, but Samsung swore up and down that it didn't exist. They were lying through their teeth of course and did so in an attempt to encourage higher sales of the original S5 before following with the derivative model. Understandably this deception angered and outraged a lot of Samsung customers who relied upon Samsung's assurances that there would be no S5 Prime model.
As a result of the marketing blunder, the S5 Prime was later claimed to be an entirely different phone and only for far Eastern sales e.g. Korea, China. The net result being that it has a substantially different firmware and seems destined to be something of an orphan like the 900H. Quite likely there were never be an extensive number of custom ROMs available to it now that it has been artificially separated from the original S5 model.
You can root all of the models, but don't count on English (or Gaelic) support for the S5 prime. Or any assurance of many custom ROMs to come. A stock Android will already do most of what an IOS jailbreak aspires to. Rooting is different and more extensive. It's not just the mitigation of some very anal IOS restrictions, it's full access to the OS and most of the flash (which you can't do at all on IOS, even if jailbroken).
Samsung will try to deny warranty claims if you increment the "Knox flag" which is a recent DRM invention. It's tripped if you try to install a custom kernel, recovery or use an older rooting method. However most warranty claims are not made directly with Samsung. Most claims are made through carriers, most of which do not care about Knox. Not to mention that Samsung is running afoul of EU law in trying to deny warranty claims as there is a directive to not arbitrary deny warranty due to rooting.
In any event, you would be wise to stick to a 900F model. If you get a prime, it will always be a struggle to make it do what you want as it will never be fully supported outside of the Asian market.