If you do book through a third party agency like Kiwi, they will lock you out from ability to check-in or check your reservation directly on Ryanair website. What they do is they change your email address to their internal address to purchase your ticket and before the day of your departure, they check in for you and send you your boarding pass in email attached as PDF. If you are EU citizen and you would be eligible to get mobile boarding pass, forget about it. You will be locked out from this feature and possibly many other useful functions.
I wrote review on Trustpilot, which is the website you should not take seriously and even though I wrote a negative review with complaint about their check-in process it did not get truthful acknowledgment but some deceitful dance around it.