One of the basins in newgrange is definitely granite ,which means it took a hell of a lot of work , in "Archaeology Ireland" winter 2003 Gerry Bracken describes how he made a replica in polystyrene and came up with a formula he believes the masons used . what beauties they are.
I've got something that I describe as a basin - but it's nothing like that stone. It's a flag, set horizontally (or *was* set horizontally) with angular corners and a flattened-circle shape three inches deep and a foot and a half across on the upper face. It weighs about a hundredweight and I hope to move it to safety in a wheelbarrow one evening.