The Scots-Irish in Pennsylvania and Kentucky by Billy Kennedy Causeway Press/Ambassador, £9.99 in UK

Book Service: To order this book and have it sent directly to your home or office, call The Irish Times Book Service at 1850 30 60 60

A few years ago, some American folk musicians were playing at a loyalist social club in Belfast. The crowd was polite, but largely ignored the flute and banjo players, concentrating on chatting and drinking, until the flautist played an old Appalachian tune which got a response which astonished him. When he reached the chorus, the crowd bellowed: "Hello! Hello! We are the Billy Boys/ Up to our necks in Fenian blood, surrender or you'll die/ For we are the Billy Billy Boys!"

That example of the Ulster Presbyterian diaspora meeting its roots is quite rare. Unlike that of Irish Catholics, the assimilation of the Scots-Irish emigrants from these shores into American society was total. There is a simple reason for this: the 250,000 Ulster Presbyterians who started leaving in large numbers from 1717 played a vital part in the American Revolution, helping to shape what it meant to be American.

The Irish Catholic diaspora, on the other hand, started to arrive in large numbers over a century later. By then, the rules had been written. Refugees from the great Famine were classified at the same social level as emancipated blacks, restricted to manual labour or cleaning the emerging cities.

This book, the fourth in Billy Kennedy's series, tells a more triumphant story. So successful were the Scots-Irish at shaping America that they are largely forgotten by their northern Protestant relatives. No fewer than 13 US Presidents were of Ulster Protestant stock. The legendary figures of Davy Crockett and Sam Houston shared the same lineage as America's balladeer, Stephen Foster. Bluegrass music, the direct precursor of Country and Western, was one gift to American culture, while on the other end of the musical scale the US national anthem, The Star Spangled Banner, was written by yet another Ulster descendant, Francis Scott Key.

The non-conforming Presbyterians, Baptists and Quakers, many of whom left Ireland in reaction to the penal laws, bequeathed to America its laws of separation breaking the bond between (Anglican) church and state, a vital recognition of religious difference, and ultimately freedom of speech. The contribution by these people to the US Bill of Rights was underpinned by the many Scots-Irish who supported the anti-slavery movement. When the 13th amendment to the US constitution abolished slavery in 1865, it was signed by President Andrew Jackson, "of County Antrim stock".

So far, so impressive. Billy Kennedy's research into the founding fathers of Pennsylvania and Kentucky, like his earlier surveys of the Scots-Irish of Tennessee, the Shenandoah Valley and the Carolinas, manages to be surprising and entertaining. These are exercises in popular, even populist, history, aimed at a diaspora market. There are lists and lists of names, very brief biographies, along with the Ulster county of origin. There is also a peculiar obsession with the precise religion of the subject: "President Nixon was a Quaker", "Ulysses Grant was a Methodist".

There is no analysis of why a disparate collection of Ulster planters, uprooted by famine and religious persecution (by Anglicans), became so successful in the virgin plains and verdant valleys of the New World. There is some sense, however, of the hatred they felt towards the English. When the time came to boot out King George's men, many must have been motivated by revenge as much as by the stirring words of Thomas Jefferson and Tom Paine. The connection seems obvious between the Ulster Presbyterians who fought and died for American independence and their cousins "back home" who joined the United Irishmen. Belfast was, after all, the site of the first US consulate in Europe.

Perhaps Kennedy is uncomfortable with this. The first chapter helpfully introduces Northern Ireland to (presumably) American readers thus: "Northern Ireland is an integral part of the United Kingdom with a population of 1.6 million." Kennedy also omits to mention that the three US Presidents who have faced impeachment proceedings are of Ulster Stock: Grant, Nixon and the current occupant of the Oval Office, and its adjoining Map Room, Bill Clinton.

John O'Farrell is editor of Fortnight magazine