On the face of it, you'd be hard pressed to find a connection between the work of monks in early Christian Ireland and a computer scientist in TCD - but a link there is. For more than a decade, Dr Dan McCarthy, a senior lecturer in the department of computer science, has been working on the Easter calendar and the chronology of a number of Irish annals. His work, scholars say, has transformed the study of Irish history. Thanks to the computer scientist, it's now much easier to date events. McCarthy has been able to amend faults and cover gaps in the ancient texts.

The story of how McCarthy got started on this work goes back to a day in 1968, when he stepped off the ferry at Dun Laoghaire. He'd come from Australia, but immediately felt at home. So much so, he decided he never wanted to leave. Although he'd come from Perth, the society in which he had grown up was almost completely Irish. His parents were part of the Irish diaspora - they'd moved first to London and then, in the 1940s, on to Australia to join other family members. Growing up, McCarthy, who was educated by both Loreto nuns and Jesuits, was surrounded by people born in Ireland. One of the first things he did in Dublin was to enroll in an Irish-language class at Trinity.

He came to Ireland on a scholarship, to work on a PhD in TCD. Although his undergraduate degree was in engineering, he was interested in maths and switched his focus to work on relativity. His work, he recalls, involved vast amounts of algebra, and for this, he used a computer. "I then decided I was more interested in doing a postgraduate degree in computer science, so I did my PhD in symbolic computations, using computers." His interest in computing remains mathematical rather than engineering, he says.

In 1971, he became a junior lecturer in the computer science department.

It was as a result of an international conference on information technology, hosted by TCD, that McCarthy first began working on old manuscripts. As part of an exhibition on the history of computation, he did some work on the argument about the date of Easter.

Later, UCG's Dr Daibhi O Croinin, with whom he had collaborated, sent him a photocopy of a table that he had discovered in Padua, Italy. This document listed the dates of Easter for 84 years during the fifth century. Using a computer, McCarthy was able to work out how the table had been calculated. This way of calculating Easter had been abandoned in 716 and was lost to scholars for 1,200 years. Until the McCarthy/O Croin in collaboration, scholars were unaware of how the calendar in early Christian Ireland worked.

At this stage, McCarthy was hooked. His next project was on the Irish annals, which date from the fifth century and run to the end of the 16th century.

The annals are, he says, exceptional in western Europe. "We have records of people living and dying in Ireland from the fifth century. You won't find anything like that in Britain or France and Germany. Their records start much later. The Irish annals, for example, have records of solar and lunar eclipses from the late sixth to the early 12th centuries. These are not copies - they are Irish records of Irish observations. They managed to record every total eclipse on the island of Ireland for over 600 years. It's an astonishing achievement."

Although the annals represent a wonderful record of Irish history, they do present problems. McCarthy has worked on the annals of Ulster, the Chronicon Scotorum, the annals of Clonmacnoise and the annals of Tigernach. Different annals contain different dates for the same event - the arrival of St Patrick in Ireland or the birth and death of St Brigid, for example, are particularly controversial. This has made scholarly work difficult.

Rather than recording the date as a year - 510, for example, the annalists recorded only the day of the week on which the first of January fell. To solve the puzzle, you had to know in which year Tuesday, say, was New Year's Day. "This, and the fact that they had numbers (which were often corrupted) embedded in them, attracted me to the annals," McCarthy comments.

"Because of the presence of astronomical records in the annals, I realised that these people were numerate and competent at chronology, so I thought there must be some property in this method of dating that had been missed." He was right. "I discovered a mechanism based on the number of days in the week and the number of days in the year, which enabled me to restore order to the Irish annals. We now have a uniform chronology for all of the annals."

In the last four years, McCarthy has completed 1,178 years of the annals and has 400 years to go. Although he's only able to work on them during the summer, he finds they've taken on a life of their own. "What started as an interest has taken over my life," he says. It is, though, immensely satisfying. "I greatly enjoy working in a numerical way in the literary world."

Because of his work on the annals, McCarthy has become involved in what he calls "adjacent issues". Nowadays, historians and textual scholars are in regular contact on issues of concern to them. Many of the early record-keepers were, in fact, great computer scientists, McCarthy believes. However, the system they used to date events was elitist. "They used a minimal amount of information which allowed people who were trained to look at records." Some historians, he says, have dismissed the annals as being hopelessly confused. Their problems is that they don't know how to use them. "For people who understand the system, it's easy to compute. It appears to me that those who wrote the annals wanted to control their knowledge."

According to McCarthy, a major benefit of working on the Irish annals is that many of them are stored in the TCD library. Although he mostly works on published copies, being able to refer to the originals is a bonus.

Factfile

Education: 1954-62: St Louis' Boys College, Perth, Western Australia. 1962-67: University of Western Australia (bachelor of electrical engineering). 19681973: TCD (PhD in computer science)

Family: married to Bridin, a psychologist

Children: two boys and a girl - an adventurer, an archaeologist and a student

Interests: cycling, sailing and traditional music

Holidaying: camping anywhere in western Europe

Annals chronology: www.cs.tcd.ie/Dan.McCarthy/ chronology/synchronisms/annalschron.htm