Mr Buzek received 555 votes. The other candidate, Swedish MEP Eva-Britt Svensson, received 89 votes.
Mr Buzek, a former leader of the Solidarity movement that led to the collapse of communism in Poland 20 years ago, is the first representative from a former eastern block state to hold a senior post in one of the EU institutions.
The election of the President was the first time Ireland's new MEPs cast their votes.
Later, the newly-elected MEPs will move on to voting for Parliament vice-presidents and committee chairs.
But the 736-member assembly will not elect the head of the European Commission.
Unusually, a vote to re-appoint José Manuel Barroso as president of the European Commission has been deferred until September at the insistence of the socialist group.
The Parliament will go into recess at the end of the week, starting work again on 31 August.
