The Irish Times is a stripling, of course.

The Herald had seen out three-quarters of a century (and a year) before Dublin's Protestant middle class acquired what Terence Brown calls their "primary journalistic expression and defender" in 1859. The Belfast Newsletter (born 1737) and the Press And Journal (1747) are older still. London's Times, the Scotsman, Guardian and Telegraph were all in the game before the Ascendancy raised a voice on behalf of "the British connection".

The fact need not count for much. Though besotted with the word "historic", newspapers come and go without pausing over posterity. They exist, by definition, in the moment. Their identities are shaped less by noble prospectuses than by the ever-changing cast of those who make and buy the paper. Tradition, for this industry, is easier to invoke than identify.

In the 150 years traced by Terence Brown the Irish Times has altered as Ireland has altered. The lion of empire, still roaring amid the 1916 Rising, long ago came to terms, sometimes uneasily, with political and social reality. Once conservative, it is firmly liberal. Once British-and-Irish, it hides no sub-text in its masthead. Imperialism's recruiting sergeant now extols the European Union. The rebirth of a nation has forged the modern paper. That, as much as anything, is the theme binding this history together.

When the Irish Times was founded it was still possible, even credible, for 3,000 landed Protestant families to dominate a country's affairs. It was still deemed natural, indeed, for such families to look to London for leadership or advancement, and to lands across the Irish Sea as homes from home. What they lacked in numbers they made up for in political and economic power.

In essence, the Irish Times represented a class that in 1800 had been bribed to abolish Dublin's parliament. As in Scotland, Unionism embedded itself thereafter, but Ireland's religious complications gave another edge to every argument. After all, a minority faith owned and ran the country. Before partition, the 100 miles between Dublin and Belfast was no great political or religious distance for those who controlled the majority. The Irish Times was in no doubt as to its loyalties.

The inept Fenian Rising of 1867 gave the paper an early chance to define itself. The villainous rebels had been crushed easily, the paper was pleased to report. It contended, with no evidence produced, that they had been led by outside agitators, doubtless American or English, while peasants "to a man" had refused to become involved. After all, these souls were simple, loyal, picturesque and content. Stereotypes, as Scots might recognise, were often home-grown. Their purpose was to secure the Union.

Until at least the middle of the 20th century, the Irish Times was essentially a Dublin newspaper with a modest circulation. Brown's earlier extracts read, often enough, like snatches from a private correspondence. For all that, by the eve of the First World War neither the paper nor its readers could ignore the world entirely. Entrepreneurs were demanding their share of empire's spoils. Land reformers, suffragists, Gaelic revivalists, trade unions and nationalists were competing to be heard.

During the great Dublin lockout of 1913, when the city's working class made a direct challenge to the power the Irish Times existed to serve, the paper did not hesitate. It was, writes Brown, "unabashed, and rebarbatively so, in its support of the employer's interest even as the immiseration of Dublin's army of low-paid workers and of their families intensified". Nevertheless, there was a hysterical note to editorials directed at the union leaders Jim Larkin and James Connolly. The words reeked of fear.

Larkin was the "Great Anarch"; the workers' movement an octopus draining Dublin of "its very life-blood". According to the paper, the Transport and General Workers' Union "must either crush or be crushed". Finally, the voice of the Establishment quavered: "The very existence of Dublin is at stake."

That possibility became real three years later, when British artillery levelled much of the town centre as it crushed the Easter Rising. The Irish Times's offices then were only yards from the General Post Office where Padraic Pearse proclaimed a republic. Yet while the fighting was going on, the paper gave full support to the declaration of martial law. When the dust began to settle, it called on the state to cut the "malignant growth" of armed nationalism from "the body of Ireland" and endorsed the executions of ringleaders.

In that moment, nevertheless, Unionism ended for the Irish Times as it did for Ireland. The first editorial after the Easter events asserted that the Rising would "pass into history with the equally unsuccessful insurrections of the past". The paper, in that cliché beloved of leader writers, was on the wrong side of history. Ireland was set on the road to independence even as it began to be understood that Unionists were split, north and south. Without Ulster's support the southern branch was helpless.

The Irish Times did not cease to be conservative after the birth of the Free State, nor did it abandon its "Victorian disdain for concentrations of working-class people". By the 1930s, nevertheless, it had come to recognise in Eamon de Valera a politician, "republican" or not, conservative enough to satisfy its strictures. All over Ireland former rebels and reactionaries were beginning to adapt to one another.

In the years since the paper has had to deal with three formidable facts. However secular it might seem, Ireland is a Catholic country if it is anything. The island itself is meanwhile divided in a way that would have appalled 19th-century editors. The country called Ireland is, moreover, a republic, one that has only recently begun to feel at ease in its relations with the British crown. Given the origins of the Irish Times - a paper that did not appoint a Catholic editor until 1986 - a tricky course has been steered.

Brown, as a former Trinity College professor of Anglo-Irish literature, does not neglect the contribution of writers to the paper, from Myles na gCopaleen to John Banville. He is good, if plodding, on the domestic Irish political context, astute on journalism and the Troubles, and alert to weaknesses, not least the Irish Times's indulgent treatment of the sleazy Charles Haughey. What remains unclear is the book's sub-title. What is a newspaper's "influence"?

It does not cause governments to rise or fall. It does not, contrary to boasts, "form" many opinions. A paper is more likely to follow than to create fashions. Its editorials might sound like court judgements, but they amount to no more than arguments and opinions. The Irish Times has adapted to changing circumstances, it seems, as often as it has exerted influence. That might be the point.

Like every title, the paper has had a troubled 21st century. These days, the former "Old Lady of D'Olier Street" - a place that seemed to exist in a haze of ink, smoke and remembered voices - occupies a modern glass pile on Dublin's Tara Street. Yet her presence extends, even now, throughout the culture and recent history of her country. You could only measure the true influence of the Irish Times if it ceased to exist.