Taoiseach (Irish prime minister) Leo Varadkar said the move was "an act of solidarity with the UK".
Britain has already expelled 23 Russian diplomats, with Moscow retaliating with a tit-for-tat move.
"Today's actions make the United States safer by reducing Russia's ability to spy on Americans and to conduct covert operations that threaten America's national security", said a spokesman. A British policeman who was exposed to the nerve agent when he responded to the attack on the former Russian officer has now been released from treatment.
"With these steps, the United States and our allies and partners make clear to Russian Federation that its actions have consequences".