Best seafood restaurant in Dublin?
I can't really think of a central place that specialises in seafood. Richard Corrigan had Bentleys on Stephens Green but had to close it.
There are a few places in Howth and Dun Laoghaire, but these are well outside of the city.
The best seafood is in the best restaurants generally - all will have good options on their menus.
Since you mentioned fruits de mer... if you are looking for a traditional French style you could try Les Freres Jacques on Dame Street. They serve oysters and lobster from an old fashioned tank. Freshness guaranteed! The food is solid but not at the level of the city's best.
A little further from Stephens Green is The Lord Edward. I have never been myself but it offers a huge choice of classical seafood dishes.
This is a pretty good list of Places throughout Ireland with a seafood emphasis.