Dennis Oppenheim is one of a generation of artists who came to maturity around the mid-1960s in the US. It was the beginning of a period that has been described as "an extended free-for-all" in art and which lasted well into the following decade. Against a background feeling that a central line of inquiry in Western art had simply come to an end, artists threw away the rule-book and began to re-define what art might be.

While the general conclusion was that art might be anything, that "anything" tended to come under the wide umbrella of Conceptualism.

In a sense, this was a delayed revolution, one that began with Marcel Duchamp's invention of Conceptualism in the second decade of the 20th century.

Though from then on Duchamp was there as a brooding presence, it wasn't until the mid-1960s that artists started systematically picking up on the significance and possibilities of his ideas.

Hence the notion that while the first part of the art of the 20th century is dominated by Picasso, the second half belongs to Duchamp. He suggested that art could be anything so designated by an artist, that it could be defined by chance or arbitrary constraints, and he shifted the emphasis from the art object to the idea.

Oppenheim, who was born in Electric City, Washington, in 1938, moved to New York in 1966 and is still based there. Though he had studied painting (in Honolulu and California), he had found himself doing more sculpture than painting.

"Some uniform things seduce artists out of painting and into sculpture," he explains. "You're freer, you can engage in a more ambitious and aggressive way with spatial situations, and you can work bigger, particularly with sculpture as it became re-defined."

He was intimately involved in this redefinition himself, and IMMA's exhibition, Land and Body, documents 16 of his works from the 1960s and 1970s, an enormously concentrated period of activity for him and others (an extensive video programme fills out the overall picture of his output then and since).

To say that Land and Body for the most part documents rather than consists of his work is accurate in the sense that often documentation is all that remains of it.

Part of his rationale, in the spirit of the time, was to take art out of the gallery and into the real world. He came up with the idea of "activating" areas of the world outside, so that they become, temporarily, "art areas".

It was, he says, "part of the deconstruction of the object. That's what we were doing, deconstruction, although as I recall the word wasn't actually used at the time. Terms like dematerialisation were current. It was attractive to a lot of artists because it defied a lot of entrenched theoretical positions, it was radical, yet it was simple in form."

His Gallery Transplants involved marking out the dimensions of an art gallery in the open space - in the snow, on one occasion, so that the work disappeared with the spring thaw. In his Cancelled Crop, wheat cut from a field in the shape of a huge X was set aside - cancelled. "As the space became more important, the nature of the phenomenological exchange involved in the artwork changed. The viewer was brought directly into the conceptual orbit of the work."

All the activity of the time was accompanied by a bewildering plethora of terminology. Rather than settling for an all-encompassing Conceptualism, artists and commentators came up with Process or Anti-form Art, Earth or Land Art or Earthworks, Systems Art and Performance Art.

Human nature being what it is, artists could be extremely territorial about their particular intellectual domains, and some tended to settle defensively within them.

Oppenheim has always been characterised by a tendency, or perhaps an ability, to move on, and as the title implies, Land and Body chronicles a shift from one to the other. "I called the summer of '68 the Summer of the Hole in the Ground, because it seemed everyone was out digging holes. Getting out into the earth opened up a huge artistic territory, but the territory quickly became explored - the rampage was on - and exhausted."

Body Art, an enormously influential initiative, involves the idea of the body as the site for art. Many of Oppenheim's initial pieces in the area involved simple actions documented on video. It was a prolific time: "It was concentrated. I wanted to do something pretty much every day."

In Material Interchange, a piece of one of his fingernails is deposited in a crack in a floorboard, and a wood splinter pierces his skin. Much Body Art involved exposing the body to the risk and actuality of pain and injury - Chris Burden famously had himself shot in the arm, and that had to hurt.

In his catalogue essay for the show, American art critic, Thomas McEvilley, plausibly suggests an intended irony in Material Interchange, directed at the more spectacular, visceral forms of Performance. Not that Oppenheim wasn't prepared to suffer for his art.

In Reading Position for Second Degree Burn he exposed his torso to burning sunlight for five hours, leaving the pale outline of a book on his chest. He used sunlight to paint his skin, as he put it. Another piece records his reactions to an approaching tarantula.

Oppenheim took an unexpected turn in 1974 with his use of mechanical puppets, "surrogate performers" which took the place of the artist's physical presence.

His first "surrogate" piece, Theme for a Major Hit, features a puppet - with his face - performing Body Art type movements. He sees it as a way of dealing with the object in the context of a general trend towards dematerialised expression, in which "less and less is retained of the object".

Though his approach is firmly theory-based, almost severely cerebral, he is less doctrinaire than some Conceptualists, such as Joseph Kosuth.

He felt less constrained from crossing boundaries than some artists. "I was never that interested in signature work. I didn't feel that strongly about anything that I wanted to do it for the rest of my life, though I think the earthworks offered infinite possibilities. That's sort of petered out: some artists have died, others have moved on to other things. I think in a way I haven't really left it, I've sort of pulled it along with me."

It's a subject that prompts some further thought. "If I've seemed scattered, seemed to cover a wide range, it's not because I was lost, it's because I wasn't satisfied. In a way you're concerned as an artist never to be satisfied, you're always uprooted, in a psychopathic way, detached, never very comfortable. To feel accomplished in a certain area and stay there is not necessarily a good thing."

From our perspective there is a certain earnestness, an almost evangelical quality to that era of Conceptualism which was supplanted, and to some extent subverted by its more recent manifestations in the US and Britain.

"The fact is that the issues of 1968 are no longer pertinent in the sense of being a continuing campaign. What was radical then isn't now. The desire is the same, the quest is the same, but in many respects the pieces are more traditional than they were in 1968. That's sometimes the way.

"I think Land Art was probably the last heroic American art movement that really pushed the boundaries in a way that continues to intoxicate both the modern and post-modern world."

OPPENHEIM is, as ever, prepared to move on. He sees the area of architectural sculpture as promising. "Well, it's an area where I can at least fantasise about seeing some movement - in an art world we've seen as being fixed, as not having any fractures. Perhaps there's a certain pressure because public art has been so horrible for so long, and now there is some interesting dialogue between architects and artists, some movement that might be construed as a new formation."

This potential new formation entails collaboration from the earliest stages of design, from underlying philosophy to finished structure. It is, inevitably, an area he is involved in.

"I tell myself I'm still an artist with a studio practice . . . in fact I'm working a lot with architects. In the past the artist's relationship with architects was that they wanted you to decorate the lobby. It's quite a development to find yourself working on the philosophy of the building . . ."

Does it generate anything like the same energy and excitement as the explosion of possibilities in the late 1960? "Actually I don't particularly look forward to it," he sighs. "I'd much rather decorate the lobby and get out." But, as another artist, Jasper Johns, once said: "Once an idea is given you're stuck with it".

Oppenheim's tone, tired but dutiful, is that of a man stuck with his commitment to new ideas. "It's a question of becoming accustomed to this new self, as you come around to the final stretch . . . but then it's always the final stretch, and you always keep going."

Dennis Oppenheim: Land and Body is at the Irish Museum of Modern Art, Kilmainham until April 22nd