Leo Donofrio has provided an excellent article on the Natural Born Citizen rule from the US Constitution,
the grandfather clause and precedents involving US Presidents. Donofrio examines James Buchanan, Andrew Johnson, Woodrow Wilson, Herbert Hoover and Chester Arthur.

“This essay will discuss the eligibility of every President who had parents born abroad.   As long as the parents had the future President on US soil after they became citizens, then that person is a natural born citizen.
Every President born before the adoption of the Constitution was eligible because of the grandfather clause of Article 2, Section 1 :

No person except a natural born Citizen, or a Citizen of the United States, at the time of the Adoption of this Constitution, shall be eligible to the Office of President;

JAMES BUCHANAN

The first President we must examine then was James Buchanan, 14th President of the United States.   He was born on April 23, 1791 in Mercersburg, Pennsylvania.  He just missed  out on the grandfather clause as the Constitution was adopted on September 17, 1787, by the Constitutional Convention in Philadelphia.   Buchanan was also the only President from Pennsylvania and the only President never to marry.

Both his parents, James Buchanan and Elizabeth Speer, emigrated to the United States from Ireland in 1783.  It was an interesting year for the United States as the Treaty of 1783 was signed between the US and Great Britain.  Colonists chose to be United States citizens and by virtue of the Treaty, Great Britain recognized those former subjects as United States citizens.

Before the Constitution, United States citizenship was conferred on citizens by the States.   When the Constitution was ratified, each citizen of a state became a citizen of the United States.  No formal naturalization was needed.

On June 21, 1788 the Constitution was ratified.  The Buchanans were citizens of Pennsylvania and therefore citizens of the United States.   When their son James was born in Pennsylvania he was therefore a natural born citizen, born on United States soil to two US citizen parents.”

Read more here:

http://naturalborncitizen.wordpress.com/