This is one of the Child Ballads (#39), originating in the Scottish Borders. It’s definitely one of the “big” songs.
The story revolves around the rescue of Tam Lin by his true love from the Queen of the Fairies. While this ballad is specific to Scotland, the motif of capturing a person by holding him through all forms of transformation is found throughout Europe in folktales. – Wikipedia
One of the versions of the macaronic (two languages) song, Mo Ghile Mear. Len Graham gives a nice introduction through the story of a grave in Seville, Spain. Here’s a bit of information about the song picked up from Springthyme:
Many Jacobite songs are found in the Irish song tradition in both English and Irish. Here Len has taken a text in English and borrowed the refrain of the Irish Jacobite song – Mo Ghile Mear by the eighteenth century Munster bard Seán Clárach Mac Domhnaill.
The maiden fair of the opening line, now old and grey, is Éire and in the song she laments the loss of her lover, Bonnie Prince Charlie, exiled across the sea in his youth. The song air is also found in Scotland. The Séarlas Óg (meaning Young Charles in Irish) refers to Charles Edward Stuart – Bonnie Prince Charlie (1720 – 1788) and Mo Ghile Mear of the refrain can be translated as – my bright hero – also Bonnie Prince Charlie.
Len added the ‘peace’ conclusion from Robert Burns’s song of 1794 – On the Seas and Far Away.
Once there was a maiden fair,
Now she’s widowed old and grey;
Her true love ploughs the salt sea spray,
Over the hills and far away.
She’ll sit down on yonder hill,
And take her pen and write with skill;
Her love she’ll raise all else above,
Her deeds she’ll praise, his worth she’ll prove.
Sé mo laoch mo ghile mear,
Sé mo Shaesar ghile mear;
Suan ná séan ní bhfuaireas féin,
Ó luadh i gcéin mo ghile mear.