"Long threatening, comes at last" is an old Irish saying. It's also a perfect description of Christy Moore, the great Irish folk singer who will finally make his American debut at the Birchmere tonight, after more than a half dozen false starts. Moore, a seminal figure in Irish music through his founding of both Planxty in 1970 and Moving Hearts in 1980, says he "had a problem with flying for about six or seven years, which I've managed to overcome. A few of the proposed visits had to be canceled for various reasons, but the main reason down through the years has been flying. I detest flying."

Since his first album in 1969, Moore's career has been split between various solo stages and the two bands.

"Right now, I'm on me own," he said last week from Dublin. "For the last two years, I've been playing completely solo. Occasionally there have been get-togethers, but they haven't been of an official or public nature. We do the odd thing together, but usually it's the social kind of thing. At this point in time, Moving Hearts no longer exist, they're finished. And at this point in time, Planxty doesn't exist, either."

Planxty in particular breathed new life into the moribund Irish music scene. The four outstanding musicians and singers (Moore, Donal Lunny, Andy Irvine and Liam O'Flynn) created an enthralling mix of traditional tunes, protest songs and patriotic ballads, all couched in exquisite acoustic grace. Years later, Moving Hearts would take things a step further, combining traditional and rock instrumentation with an increasing protest element. Both bands had a tremendous effect on Irish and English folk and folk rock, but each time Moore departed just as things were about to break, as if he no longer wanted to be contained in proven structures.

"I've always left on very good terms," he points out. "I don't have any problem working with other musicians, but at times I have felt the need to go, and the people I'm working with have understood that. It's more a need to do something different than any problems with individuals."

Actually, Planxty had first come together in the studio, working on Moore's second solo album, "Prosperous." He had been living and playing in England, having originally left Ireland when his job as a bank clerk was interrupted by a major bank strike (the County Kildare native also worked for a while on a North Sea oil rig). "I went back to record the second solo album. Everything worked so well we decided to have a try at a band and it was very successful," he says.

Planxty became the toast of the British Isles and the Continent, building a reputation in America despite Moore's inability to fly across the Atlantic. But by 1976 the group was falling apart, and even though it continued in various forms, it was never the same after Moore's departure. In particular, they missed Moore's wondrous vocals, the iron behind the velvet, in the words of one of his songs.

In the late '70s Moore became a major solo act, winning a number of polls (including one that named him the worst-dressed man in Ireland). A supporter of the Provisional IRA, he also got involved in the antinuclear power movement in Ireland and with the Long Kesh hunger strike of 1981. Not surprisingly, his solo albums have often been informed by this world view, and a sense of solidarity in national struggles. "There's a strength to be gained from looking out," he says. "It's comforting to know that it's not a lone struggle."

"As far back as I can remember, I've never been totally immersed in Ireland, even though the basis of all I do is Irish," Moore says. "I've always been attracted to music and lyrics from other places. I like to take a broader view of things, and I think it's good to have a more international approach." The songs on his new album, "Ride On," though all written by Irish writers, cover the Irish involvement in the Spanish Civil War ("Vive La Quinte Brigada," with an interesting Irish-Spanish accent from Moore), El Savador, Chile and the plight of immigrants. "I wonder how people over there will react to my singing Woody Guthrie songs with an Irish accent," Moore wonders.

His new album also contains two songs by Bobby Sands, the IRA political prisoner who starved himself to death in 1982. Sands was a big fan of Planxty and Moore. "I play quite a lot in the north of Ireland," Moore explains, "and when I'm on the road I'm always looking out for new material. On consecutive nights I was given these songs by different persons in different cities. They came directly from comrades of Bobby's in the Block."

"McIlhaton" is about a moonshine maker, and "Back Home in Derry" is a ballad about deportees. "It's the first time they've been heard by a large audience, or by the general public," Moore says of Sands' songs. "A lot of his writing has been published -- a book of poetry, a little bit of prose, but it's the first time for songs. There just aren't many Bobby Sands songs about -- three or four others." One in particular, Moore found too personal and moving. "You couldn't sing it to a public audience or even a private audience. I cracked up when I heard it."

With his first American tour opening at the Birchmere, Moore has an opportunity to meet a whole new squad of fans who consider him the most influential Irish musician of the last decade. He himself has no great sense of his contributions. "No, I'm always kind of interested to read about it," he laughs, adding, "I don't really take that very seriously. I find it very flattering, but I have more important things to worry about, like the ------- flight to Washington."