Ireland's County Donegal and Scotland's Edinburgh, both in the far northern climes of the British Isles, have been united for centuries by a common opposition to the colonizing British and the hardship of scratching out a living from the chilly air and rocky soil. They've often exchanged migrant workers across the short North Channel, and those workers exchanged songs.

Thus the folk music of Ireland and Scotland are closely related. They share a Celtic background that comes out in the lively jigs, reels and waltzes, and the fatalistic ballads about hard work, true love, the cursed British and beloved landscapes. But the relativity of these two traditions is more often talked about than acted upon, so the new Irish-Scottish folk quartet Relativity, which makes its Washington debut at the Saba Club tomorrow, is a welcome development.

The quartet pairs two famous sets of siblings. Triona Ni Dhomhnaill and her brother Micheal O Domhnaill grew up in County Donegal and eventually became the lead singers for Ireland's legendary progressive folk group, the Bothy Band. Brothers John and Phil Cunningham grew up in Edinburgh and eventually formed the instrumental core of Scotland's progressive folk band Silly Wizard.

"When you play and sing with someone from your own family," Triona Ni Dhomhnaill reflects, "there's a bond there you don't share with other people. Just by glancing over at one another while you're playing, you understand what the other one wants. There's a lot that doesn't have to be said."

Triona and her brother grew up in a house where Gaelic was regularly spoken and quite often sung. On their records, the two often give credit to their father, Aodh, and their Aunt Neili for teaching them the songs they still perform.

"We'd sing around the house pretty much any time at all," Triona recalls. "We never needed much of an occasion; any time relations would come down or people would come over, someone would start up a song, and everyone would join in. After you've done that for years, it just gives you so much in common. I can hear it in other family groups I've heard; there's such a close blend in the voices that often you can't distinguish one from the other."

Though it only lasted four years (1975-79), the Bothy Band was one of Ireland's most influential folk groups. They made four excellent albums (all rereleased in America on Green Linnet Records) that combined the solo singing with the group instrumental styles of traditional Irish music and added new dashes of rock, jazz and classical music. In this vein, Triona added the clavinet to an otherwise acoustic lineup.

"I happened to find this instrument in a music shop in Dublin one day," Triona explains. "I just liked the sound of it, and I've been playing it ever since. We worked it in as if it were a harpsichord, which it resembles. It has strings in it, so it has a certain acoustic sound, but with the punch of an electric instrument."

After the Bothy Band broke up, flutist Matt Molloy joined the Chieftains and guitarist Donal Lunny cofounded the Moving Hearts. Guitarist Micheal O Domhnaill and fiddler Kevin Burke moved to Portland, Ore., from which they toured North America as a duo. Keyboardist Triona Ni Dhomhnaill moved to Chapel Hill, N.C., where she met up with some old-time string music enthusiasts and formed the hybrid Irish-Appalachian band Touchstone.

Meanwhile both John and Phil Cunningham grew up as child prodigies in Edinburgh. John was giving violin recitals at age 7, and Phil won the Scottish Accordion Championships six times before his 16th birthday. While still in their teens, they both joined Silly Wizard in 1976. Led by gifted singer Andy Stewart, Silly Wizard proved the Scottish equivalent of the Bothy Band.

"I recognize the same special relationship between siblings in Johnny and Phil," Triona says. "There's a common bond, both on the stage and off, both in the music and in their lives. I've listened to their music for a long time and I like it a lot, so the prospect of playing with them was exciting. We've talked for years about doing something together, but it wasn't until last summer 1984 that we were able to get all four of us in one place.

"We all knew a lot of the same songs," Triona says. "And because we've been playing this music so long, we were able to cut corners and get through a lot without talking about it. Johnny and Phil knew some tunes we'd never heard, and Scottish music has different accents, different emphases, but the overall feel is quite similar."

The result was "Relativity" (Green Linnet SIF 1059), one of the year's most enchanting folk albums. Phil's Elysian melody "Gracelands" was written for his adopted home, the Isle of Skye, and the sighing blend of his accordion and whistle with John's fiddle captures the island's stark, exhausting landscape.

When John and Phil join Triona on the vocals for the rousing Gaelic standard, "An Seanduine Doite," they had to learn the words phonetically, but the result is a convincing pub sing-along. On the 18th-century anti-British song "Gile Mear," Micheal sharply strums out an anthemic march rhythm on the guitar, but Triona's harmony vocal and John's fiddle give it a fatalistic twinge.

"Nothing was really planned," Triona insists. "We got together and learned some songs over the course of a few sessions. It's just like any band: You strike up friendships with musicians, and the next thing you know you've formed a band, and the next thing after that, you've made a record and you're out on tour."