I sat in a Park Royal Coach No. 1944 in Downpatrick back in August, And I have to say, they are so comfortable.
Unfortunately I have not had the pleasure to travel in 728 yet but I hope to soon.
The DDs are my personal favourite, find the seats comfortable, but I cannot unfortunately say the same for the Irish Rail Mark 2 or Mark 3 (I never travelled in them) have not tried the Mark 4's, 2600s or 2800s yet.
When I got an ICR to Dublin on the 1035 ex Belfast in November 2022, it was interesting, but I got a bit of a sore back from the seats (and there was a queue for the bathroom onboard)!!!