The Merrion is my favourite Dublin hotel. It's the glorious Georgian building (birthplace of Wellington incidentally) and the Irish art collection. Compared to other branded hotels where you could be in any city in the world, you are undoubtedly in Ireland at the Merrion. They have an 'Art-Tea' thing they do where the dishes take inspiration from paintings on the wall - haven't tried it but sounds interesting.
The Shelbourne is a close second for me and an old Dublin classic - poor old Countess Markiewicz should have taken over the Shelbourne rather than the College of Surgeons in Easter 1916 cos the British soldiers ran across the Green up to the roof of the hotel and could pin them down from there. Its been dolled up somewhat and is all contemporary Marriott style and glamour now - if you like that mixed in with history then it's for you.
I'm not one for afternoon tea and I'd imagine the London hotels mentioned on the boards here would do an English classic best. Please report back and tell us what you thought of the hotel you picked.
A friend of mine stayed at The Merrion and enjoyed the tea, but it wasn't me so I can't personally recommend it. I was at the Westin, which was very elegant, but I did not have afternoon tea. Their breakfast was wonderful, and I imagine their tea is very nice. That said, here are some comments about The Shelbourne... sounds like a winner to me.