I have had a go at this and present the results below as an literal interlinear translation with notes. It would be great if someone more competent than I in the language (Siaron? Hendrik? Well, pretty well anyone really…) could cast an eye over it and check/correct my understanding.
A Great Thing Is Love [i.e. a powerful force]
Peth mawr ydy cariad pan elo fo’n drwm,
A great thing is love when it goes grievously, [elo: 3rd person present subjunctive form of af (I go). Why a subjunctive, you may ask, and I think it’s just because the Welsh tend to use that mood for proverbial sayings. ‘Drwm’ from ‘trwm’, ‘heavy’ but perhaps here ‘grievous’
Peth gyrrodd gryn lawer o’u llefydd i ffwrdd
A thing that drives a good many away from their places [i.e. their familiar haunts] [‘gryn’ from ‘cryn’, here just used as an intensifier]
Peth gyrrodd fi fy hunan oedd geiriau fy nhad,
What drove me myself were the words of my father
A’m mam, oedd yn garedig, a’m gyrrodd i o’m gwlad.
And my mother, who were loving, they drove me from my country.
Verse 2: now it is the father and mother speaking. They evidently don’t approve of their son’s choice of beloved, and these were those unimaginable times when children took notice of their parents in such matters…
‘Mi fynnaf gael dy gladdu a’th roddi di dan bridd
I want to see you buried and put you under the earth [‘gladdu’ from ‘claddu, to bury; ‘bridd’ from ‘pridd’, earth, soil]
Cyn cei di briodi; mi’th claddaf di, yn wir.
Before you get yourself married; I’ll bury you indeed.
Rhof dorchen ar dy wyneb a charreg uwch dy ben
Put a clod on your face and a stone (i.e. tombstone) above your head [‘dorchen’ from ‘torchen’ which is a variant of ‘tywarch’, a clod, sod, piece of turf)
Cyn cei di fartsio’th gorffyn, wel, gyda’r feinir wen.’
Before you get to march your wretched body (i.e. to the altar), look you, with the fair maiden’.
[‘fartsio’ from ‘martsio’, to march; ‘gorffyn’ from ‘corffyn’, derogatory diminutive of ‘corff’, body; ‘feinir’ from ‘meinir’: beautiful young woman, sweetheart. I take ‘wel’ to be from ‘gweld’ rather than the interjection meaning well]
Pan glywais innau hynny es gyda man–i–wâr,
When I heard this I went with a man-of-war [i.e. enlisted in the navy]
Bum hefoi am saith mlynedd heb weld na thad na mam;
Was with it for seven years without seeing either father or mother. [hefoi is a contraction of ‘hefo hi’, with it]
Saith mlynedd wedi pasio pan ddois i i Gymru’n ô1,
Seven years had passed when I came back to Wales [‘dois’ from ‘dod’]
Gan dybied yn fy nghalon fach na fyddwn i byth mor ffol.
Thinking in my little heart I would never be so stupid again. [‘dybied’ from ‘tybied’, to think, suppose]
At dŷ fy nhad mi gerddais, lle bum i lawer tro,
I walked to my father’s house, where I had been many a time
A phawb oedd yno’n llawen fy ngweld yn dod yn ô1;
And everyone there was glad to see me come back.
Awr nos a ddaeth yn brysur, a’m meddwl gyda mi,
The hour of night came swiftly, and my thought with me [‘brysur’ from ‘prysur’, usually means ‘busy’ but here ‘swift’]
At dŷ yr hogen annwyl cyfeiriais yn bur hy.
That I would make my way to the house of my dear girl most boldly [‘cyfeirio’: to direct, guide; ‘bur’ from ‘pur’; ‘hy’: bold, audacious, confident].
Evidently there was a fifth verse to the song but the singer from whom it was collected was not taught it because her father considered its contents to be too indecent. It describes the travelling hero’s return to his home, where he is met with a child, as well as a lover.