How many US presidents had Irish heritage?

There are twelve, counting the current president ,Obama, but only two with ancestors that would be likely to wear green on St. Patrick's day. John F. Kennedy was the first President of 100% Catholic Irish heritage, Ronald Reaganwas of Catholic Irish ancestry (originally O'Reagan) but his family converted to Protestantism after moving to America.

There had been several Presidents of "Scotch-Irish", i.e. Protestant Irish ancestry, mostly from Northern Ireland, which is now a separate country from Eire and whose people have quite different ethnic backgrounds and languages.

The first was Andrew Jackson, followed by James K Polk, James Buchanan, Andrew Johnson, Chester Arthur, William McKinley, Woodrow Wilson, Harry S Truman ,Bill Clinton and Barack Obama on his mother's side. Both of Jackson's parents were actually born in Ireland, as was Chester Arthur's father.