Numerous times, is 75% as said, same discount on Stena Line.
No problem at all buying at the IF desk in Holyhead before departure, not uncommon for them to get priv users at all, more Irish than UK I imagine.
I am sure there wouldn't be any problem in Pembroke either but it is 20 years since I was there.
I am not sure how you would get one in advance remotely, they might sell you one via the phone booking line but the sailings are very rarely full. Ulysees and Isle of Innishmore almost always have space, Swift can sometimes be full in high summer season but even then they can usually find allocations just before boarding. Epsilon doesn't take foot passengers under any circumstance.