It doesn't matter if you have made this booking via a third party. As long as they have provided you your PNR, you can use that PNR to login to your reservation on Ryanair website, and buy priority boarding (if still available) right there.
You will also have another opportunity to do so, when doing the online check-in on Ryanair's website. They will try to sell you plenty of things during the check-in process, amongst them will be priority boarding (if still available).

Priority boarding does sell out, but I believe it's very rare. That said, in your case it's definitely better to buy it sooner than later.