PJ Curtis, the song is older than the '70s.

Mary O'Hara writes in her autobiography, The Scent of The Roses, regarding her recording around the time of her husband's death in 1957..."In those days the Clancy Brothers had not yet become famous. They had started their own recording company: Tradition, and wanted me to make a long playing record for them. Being under contract to Decca, whose records in the USA were issued under the "London" label, I felt I could not do so. But when they persisted, Richard approached Decca and persuaded them to allow me to make the one album for Tradition. By the time I came to make this record, 'Songs of Ireland', Richard had been dead a few weeks.....About a month after Richard's death, I was ready to leave the Long Island apartment. Before doing so I burnt all my letters to Richard.
It was more convenient for me to stay in the city while I was recording 'Songs of Ireland'...
While the Tradition album, 'Songs of Ireland', was available only in the United States, the two Decca albums, 'Songs of Érin' and 'Love Songs of Ireland', were being manufactured and released in half a dozen countries in Europe, as well as South Africa, Australia and New Zealand. ...
With plans for going into a monastery firmly established in my own mind, I decided to record a few more long-playing albums before finally giving up singing forever. My repertoire of songs in English and Gaelic was comparatively large and I hoped, if possible, to make one album of Scottish songs because of my attachment to that country..."
She goes on to describe a concert tour of Australia, recording a number of programs on Radio Éireann, an appearance on the Ed Sullivan show with the Clancy Brothers and Brendan O'Dowda, and the final recording of tapes before entering the convent she had chosen at Stanbrook.
..."Towards the end of 1961 Ireland was preparing to set up its own television station. It opened for the New Year on 31 December. I was asked to take part in the opening night, which I did, doing a number of songs accompanying myself on the harp. I never saw that programme, as I was in Africa that night visiting my brother..."
The next time she saw her brother was in 1978, when he appeared in the television studio that surprised her with "This Is Your Life".

The Quiet Land of Erin was recorded in those sessions before entering the convent. It is available on the current CD called 'A Song For Ireland' on Shanachie.

Mary explains how she entered Stanbook in 1962, starting a new chapter of her life, leaving singing behind with no regrets..."Over the next few years, beyond the walls of my sequestered world, the saga of the "Mount Street tapes" dragged quietly on, puncutuated by long intervals of silence. Eventually, it was decided at Stanbrook that I had to listen to the tapes and judge for myself. As they could not be played at the monastery I had to go to London to hear them under proper studio conditions. Sister Raphael accompanied me and the result of the trip was that in 1972 I signed a contract with Emerald Records of Belfast, whose records were manufactured and distributed by Decca. All the songs on the original "Mount Street" master tapes have been included on three albums, now available in many countries."

She spent 12 years in the monastery, and, after illness, in 1974, was advised that she should leave the order so that she could regain her health and vigor. She was then invited to appear on the Late Late Show on Irish Television.
"... I sang 'The Quiet Land of Erin', which had become my theme song before I went into the monastery. I had not planned to return to singing so soon... Several times during the next two or three years I recorded songs for inclusion in his [Ciarín MacMathúna] tremendously popular Sunday morning programme, 'Mo Cheól Thuú'. There is now a long playing album of that title on the Gael-linn label."
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