Fiddler's Companion was a good suggestion, Peace. Here's what I came up with there:Result of search for "loch lein":
CAVES OF CONG/KONG, THE. AKA and see "The Red Haired Man's Wife," "Loch Lein," "Thios ag Beal Bearnais." Irish, Air (3/4 time, "with feeling"). D Major. Standard. AB. The air, for which there are no words under this title, is well known (both air and words) as the song "The Red Haired Man's Wife." The village of Cong is in County Mayo, close to the shores of Loch Corrib, in County Galway. Nearby are numerous caves that are historically and geologically significant. The Pigeon Hole, Ballymaglancy cave which has stalactites and stalagmites, Captain Webbs Cave, Kelly's Cave, Lady's Buttery and Horses Discovery caves in the vicinity of Cong town are mostly accessible. The Giant's Grave cave near Cong was a megalithic burial chamber and nearby at Nymphsfield there is one of a number of stone circles in the area. O'Neill (1850), 1903/1979; No. 161, pg. 28.
LOCH LEIN. See "The Red Haired Man's Wife."
RED HAIRED MAN'S WIFE, THE [2] ("bEan An Fir Ruad" or "Beann an Fhir Ruaidh"/"Bean an Fhir Rua"). AKA and see "The Caves of Cong," "Loch Lein," "Thios ag Beal Bearnais." Irish, Air (3/4 time). Ireland, Munster. D Major/Mixolydian. Standard. AB. O'Sullivan (1983) states this well-known melody is perhaps the most celebrated of the 9/8 meter airs used for Irish folk songs (though the versions in the popular collections are noted in 3/4 time). One edition of the words is to be found in "Cathal Bui" (Breandan O Buachalla, 1975) under the title "Thios ag Beal Bearnais," attributed to Cathal Bui Mac Ghiolla and also to Riocard Bairead. Neither of the two versions printed in Stanford-Petrie (Nos. 115 and 1140) are the same as Bunting's verison. The melody also appears in Poets and Poetry of Munster (1849). See also "The Roving Pedlar" [2] for another variant. An English adaptation of the song begins:
***
A letter I'll send by a friend down to the sea shore,
To let her understand I'm the man that does her adore.
And if she'd but lave that slave I'd forfeit my life,
And she'd live like a lady and ne'er be the red-haired man's wife.
***
Sean Ó Boyle (1976) relates a story told by the 19th century Tyrone novelist William Carleton, who recorded that his mother was once asked to sing the English version of "Bean an Fhir Rua." She said, "I'll sing it for you, but the English words and the air are like a quarelling man and his wife--the Irish melts into the tune but the English doesn't." "An expression," states Carleton, "scarcely less remarkable for its beauty than its truth." The title appears in a list of tunes in his repertoire brought by Philip Goodman, the last professional and traditional piper in Farney, Louth, to the Feis Ceoil in Belfast in 1898 (Breathnach, 1997). Source for notated version: Bunting obtained the melody from another collector, George Petrie, in 1839. Roche Collection, 1982, Vol. 1; No. 31, pg. 16. O'Sullivan/Bunting, 1983; No. 83, pgs. 126-128. Stanford/Petrie (Complete Collection), 1905; No. 359, pg. 91.
Also note the crosslinks I put at the top of the thread.