Paddy Moloney, the pint-sized piper for the Chieftains, is becoming a fixture in the world of film. His entry came several years back, when the group's traditional Irish music was used for the Oscar-winning "Barry Lyndon" soundtrack.

In the last year, Moloney has completed four more scores, with the biggest -- the $18 million production of Morgan Llywelyn's best-seller, "The Lion of Ireland" -- still to come.

"The Year of the French," written for a six-hour Continental mini-series, is currently the number one folk album in Great Britain. The Chieftains, who will be at the Kennedy Center Saturday, will perform it with various symphony orchestras here and in Europe.

Moloney has also done soundtracks for the Richard Burton-Vanessa Redgrave film version of "Tristan and Isolde;" an upcoming PBS telecast of J. M. Synge's "Playboy of the Western World," and "The Grey Fox," a very successful Canadian film.

"Lion of Ireland," the 11th century period piece about the first high king of Ireland, Brian Boru, starts shooting in mid-July. "I'm going to compose quite a lot around the type of tunes we think existed then," says Moloney. "A lot of the ancient music of Ireland wasn't written down. I was reading the script on the plane coming over, and the latest one has the band written to appear in it. So we're not just going to do the music, we're going to be big film stars, as well."

Moloney and his pals will get to town a few days early for a private Capitol Hill performance for House Speaker Thomas P. (Tip) O'Neill Jr. in conjunction with a new annual O'Neill scholarship at Boston University sponsored by Guinness. Moloney hopes to see some other well-known Irish faces there, including Sen. Edward M. Kennedy (D-Mass.) and, possibly, President Reagan, who's said to be a descendant of Brian Boru.

"The last time we gave a private concert was for the pope," Moloney laughs, "so we'll see how Tip takes it. He seems to be a great guy."

Moloney's also been recording as a sideman with Dan Fogelberg, Don Henley and Paul McCartney (the song, "Rainclouds," is the flip side of the smash single "Ebony and Ivory").

Next month: the Irish invade China. After that, he admits, there's only the moon to shoot for. "They'll have to build us a decent hall... and put a few pubs on it... and lots of Guinness. I hope the Guinness gets there first.