Have you ever passed a site that people are excavating, looking focused and mysteriously busy with trowels and tape at a piece of opened ground, and thought: What are they doing?

Now you may have an opportunity to find out. Achill Archaeological Field School, which runs summer courses for third-level archaeology students, has just held its first introductory course for the wider public.

Late last month, some 13 people turned up at Dooagh, on Achill, for a weekend course led by Theresa McDonald, a tutor who has long been associated with the Achill summer school, and her colleague Martin Jones, a contract archaeologist. "Archaeology is destruction," McDonald said in her introductory lecture, on the Friday night. "You can never put a site back together again."

The next morning saw us trekking in heavy rain up a track at Slievemore to its deserted village, where 74 ruined houses still stand, gables stern to the prevailing winds. "This is the killing cure," McDonald said cheerfully, as we crammed, sodden, into the tiny site hut to hear her talk about Slievemore's history. "Archaeologists have to work in all weathers, especially if they're working against time."

Killing cure or not, it was a relief to emerge to sunshine, even if the infamous Achill midges had descended by the thousand. At least it made for a swift bonding session, as insect repellent was passed round the eclectic group.

Among those taking the course were three secondary-school teachers, two forensic scientists, a schoolgirl of 16 who was taking a classical-studies option, a farmer taking a night course in archaeology, a landscape gardener, a tax accountant, a novelist researching a new book, a lecturer in computers and a man close to retirement who was looking for a hobby.

Before the instruction proper started, we pulled tarpaulin off a souterrain near an area called the Monk's Garden. A beautiful structure of lintel and large stones, set around the hollow in a fan shape, it was discovered in 1998. Finding something like this is a coup, the archaeological equivalent of hitting the jackpot.

Gary MacGinty, a surveyor, began the morning proper by giving a long talk, using high-tech equipment to illustrate the methods used to mark out a piece of ground for excavation.

He was using a "total station" that, at £10,000, is apparently the most sophisticated piece of this type of equipment on the market. Unfortunately, the procedure was far too complicated for novices like me, as was the specialised technical language, so I spent the hour or so of the demonstration being politely baffled, swatting midges and feeling very dim.

"Usually, when you're starting off, you use tapes," McDonald told us. It seemed a pity we also didn't start that way, instead of being hurled into the most sophisticated (and expensive) way of doing things. We were meant to be on an introductory course, after all.

Happily, things improved after this shaky start. We were split into two groups, the diggers and the trowellers, and got to do something at last. The diggers set to clearing the plot of land that MacGinty had marked out, while the trowellers went a few houses away to start taking back another layer of earth from the top of a lazy bed.

I commuted between the groups, to get a better idea of what was involved, starting off with the diggers. A gardening background would be extremely helpful, as you have to dig up the sod and topsoil, then go down by degrees.

There was something curiously satisfying about setting boot to spade, marking out the grassy, boggy, reedy land, then seeing the brown, waterlogged, turfy earth gradually emerge. You start wondering what it's concealing, and whether you'll find anything.

Jones, who was leading the group, told us the most exotic thing he'd found on a dig was the ankle bone of an extinct camel in an olive grove in Portugal. There was, he said, no sign of the rest of the animal.

The trowelling site was a lazy bed between two houses, which summer-school students have worked on intermittently since 1993.

To get an idea of the pace at which archaeologists work, McDonald reckoned it would take a team of 20 trained people a month to get the site to its current state.

Five of the group, on hands and knees, were carefully working with small trowels, in rows going backwards, scraping back another layer of earth and mud. The rules of excavation require you to record the findings of each layer and to work horizontally; that is, not belt down through a layer, alluring as the prospect is.

To work at the slow, meticulous pace that archaeology demands is to begin to understand how difficult and frustrating it is for people to work against the deadlines of bulldozers moving in and of sites being built on, when material is lost forever. It also goes some way towards making you realise why relatively few of our potentially rich sites have been excavated.

Some of the teachers were there to pass on their experiences - and photographs - to pupils. Since the introduction of the Junior Cert syllabus, a basic introduction to archaeology has been on the first-year history course. Pupils learn a little about the sort of work archaeologists do on digs, and they look at artefacts. A trip to a site such as the Ceide Fields or Newgrange usually follows.

The trowellers found the neck of an old bottle, a piece of decorated pottery and some flint whose worked nature indicated it had been used as a tool, all of which were bagged and recorded, to be stored temporarily at the field school. Most were found by John O'Dwyer, the man looking for a hobby for his retirement, and there was a great buzz when they were unearthed.

Back in the Monk's Garden, it took most of the day to clear the six-metre-square site. Once a fair piece of ground had been cleared, two people were set to trowel off the remains of the topsoil.

They found a couple of pieces of "black ware", made from red clay with a black glaze, which Jones told us dated from the 18th century. He knew from previous geophysics work on the site that it contained a pit, which would be revealed as the layers of soil was uncovered.

The hands-on experience of digging and trowelling prompted many questions from the group, which made for a happily organic type of learning. Before leaving for the day, the sites and some of the artefacts that had been found were photographed.

The following day's work included an introduction to planning and levelling, after which we went back to the field school's base and "inked in", making drawings of the sites, so a cycle of work was completed, even in the space of a weekend.

This course was intended to see if there was interest in a basic archaeology course. Judging by the reactions of those on the weekend, it would appear there is. The school will be planning another in the next few months.

Achill Archaeological Field School is at 098-43564. Website: www.achill-fieldschool.com