Bhí cáil ar Phroinsias os na 1950í i leith mar chraoltóir raidió le Radio Éireann, an craoltóir náisiúnta. Thosaigh sé leis an stáisiún mar scriptscríbhneoir i 1947. Go luath ina dhiadh sin chuaigh sé ar an mbóthar le hAonad Taistil Taifeadta RÉ, i bpáirt le Séamus Ennis agus Seán Mac Réamoinn, chun cainteoirí, amhránaithe agus ceoltóirí tríd an tír a thaifead ar dhiosca do chláracha faisnéise raidió, ach go háirithe sna gaeltachtaí. Lean sé air sna 1950í le téipthaifeadán, agus thar 30 éigin bliain rinne sé iliomad cláracha raidió den chéad scoth, agus chaomhnaigh sé eolas faoi chanúinti Gaeilge agus traidisiúin bhéil a bhí ag dul as.

Díograiseoir é Proinsias maidir leis an nGaeilge. Cés moite dá chuid obair raidió, a bhí lárnaithe sa teanga, bhí sé páirteach i bhfeachtais eile ar maithe léi, An Club Leabhar, an nuachtán Inniu, An tOireachtas agus Oireachtas na nGael ina measc. Ba údar agus eagarthóir cló é. Scríobh sé Scéal na Scannán (1953), Ár Scannáin Féin (1954), agus An Duinníneach (1958, le Donncha Ó Céileachair), agus rinne sé eagar ar Rotha Mór an tSaoil (1959), Seán T.: Scéal a Bheatha (1963), agus Islands and Authors (1983). Rinne sé taifeadtaí dena cainteoirí deireanacha Gaeilge in oirdheisceart Uladh agus áiteanna eile. Tar éis dó éirí as obair le RTÉ i 1983, chaith sé dúthracht le Dúiche Néill, an eagraíocht logánta staire i dTír Eoghain. Bhí sé ina ina eagarthóir ar a iris siúd Dúiche Néill: Journal of the O Neill Country Historical Society ó 1987 amach, agus bhí sé fós ag scríobh di chomh déanach le hiris 2009–10. Bronnadh Dochtúireacht oinigh sa Litríocht air ag Ollscoil Uladh i 2005.

Bhí suim riamh ag Proinsias sa cheol traidisiúnta, agus go háirithe sna hamhráin. Ina shaol mar chraoltóir bhíodh na hamhráin i nGaeilge chun tosaigh aige, agus i 1953 rinne sé na taifeadtaí tábhachta de Róise Mhic Grianna ar oileán Árainn Mhór i nDún na nGall a chéad-eisíodh ar an bhfuaimtéip Róise na nAmhrán: Songs of a Donegal Woman (RTÉ, 1994). Ach bhíodh na hamhráin Bhéarla aige leis os na 1960í amach i sraitheanna raidió mar ‘The Ballad Tree’ agus ‘The Singer and the Song’. Rinne sé cláracha faisnéise faoina chomh-bhailitheoirí ceoil Carl Hardebeck, Éinrí Ó Muirgheasa, Sam Henry agus Francis O’Neill, agus faoi amhránaithe mar Eddie Butcher, Paddy Tunney, Aodh Ó Duibheannaigh, Jerry Hicks agus John Maguire, agus faoin gceoltóir John Rea. I 1971 bhí sé páirteach le Hugh Shields, Breandán Breathach, Seóirse Bodley agus daoine eile i mbunú Chumann Cheol Tíre Éireann, agus bhíodh sé i láthair go rialta ag cruinnithe lena bhean Síle, a fuair bás roimhe. Idir 1976 agus 1987 thug sé léachtaí agus léirithe don Chumann a bhfuil tuairisc orthu sa nuachtlitir Ceol Tíre (uimhreacha 7–9, 17, 20, 26, 30–31; fáil orthu anseo). Rinne sé eagar ar a thaifeadtaí páirce den amhránaí Aontroma Robert Cinnamond don bhfadcheirnín You Rambling Boys of Pleasure: Traditional Ballads and Songs from Ulster (Topic, 1975). Bhíodh an ceol idir lámhaibh ag Proinsias leis ina altanna i nDúiche Neill, leithéid ‘Who Wrote “An Chúileann”? Was it a Benburb Man, Muiris Ó Dúgáin?’ (1988), ‘A Tyrone Woman and Her Songs: Mrs Eileen Keaney of Glenelly and Belfast’ (1992), ‘Dr George Sigerson and Those “Mountains High”’ (1994), ‘The Tunes We Played Going Away’ (2008), agus ‘David Hammond (1982–2008): Some Personal Memories’ (2009–10).

I 1990 thug Proinsias bronntanas mór fial d’ábhair cheoil – os cionn 500 píosa, maille le innéacsanna – ar an Taisce: taifeadtaí fuaime, leabhair, irisí, cláracha, nuachtáin, agus eile, agus táid anseo i gcónaí do thairbhe an phobail, cuid dá oidhreacht. Freisin, chaith sé tamaill ar Bord an Taisce.

Proinsias was a noted broadcaster from the 1950s with Radio Éireann, having joined the national station as a scriptwriter in 1947. Soon afterwards he began travelling the country with his colleagues Séamus Ennis and Seán Mac Réamoinn, making disc recordings with RÉ’s Mobile Recording Unit of speakers, singers and musicians for features programmes, and with a special emphasis on Irish-speaking districts. He continued this work from the 1950s with a tape recorder, and over a career of some 30 years produced a series of outstanding programmes. In making them he has preserved records of disappearing Irish-language dialects and oral traditions.

An enthusiast for the Irish language, Proinsias took part in various projects for its promotion, apart from his broadcasting which was centred in the language. Among them were the book scheme An Club Leabhar, the newspaper Inniu, and the cultural festivals An tOireachtas agus Oireachtas na nGael. He was an author and print editor. He wrote the film histories Scéal na Scannán (1953) and Ár Scannáin Féin (1954), and the biography An Duinníneach (1958, with Donncha Ó Céileachair), and edited the autobiography Rotha Mór an tSaoil (1959), the biography Seán T.: Scéal a Bheatha (1963), and the essays Islands and Authors (1983). He recorded the last speakers of Irish in south-east Ulster and elsewhere. After he retired from RÉ (then RTÉ) in 1983, he began a close involvement with Dúiche Néill, the local Tyrone history society, editing its journal Dúiche Néill: Journal of the O Neill Country Historical Society from 1987, and still writing for it as late as the issue of 2009–10. He was awarded an honorary Doctorate of Literature by the University of Ulster in 2005.

Proinsias had a keen interest throughout his life in traditional music, and especially in traditional song. Irish-language songs were to the fore in his broadcasting life, and in 1953 he made the important Donegal recordings of Mrs Rose Green of Arranmore island which were first issued on the audiotape Róise na nAmhrán: Songs of a Donegal Woman (RTÉ, 1994). From the 1960s he also featured songs in English in such series as ‘The Ballad Tree’ and ‘The Singer and the Song’. He made documentary programmes on his fellow music-collectors Carl Hardebeck, Éinrí Ó Muirgheasa, Sam Henry and Francis O’Neill, and about singers such as Eddie Butcher, Paddy Tunney, Aodh Ó Duibheannaigh, Jerry Hicks and John Maguire, and the musician John Rea. In 1971 he was a cofounder with le Hugh Shields, Breandán Breathach, Seóirse Bodley and others of the Folk Music Society of Ireland, and was a regular attender at its meetings with his wife Síle, who predeceased him. Between 1976 and 1987 he made presentations to the Society that are reported in its newsletter Ceol Tíre (nos 7–9, 17, 20, 26, 30–31; available here). He edited his own field tapes of the Antrim singer Robert Cinnamond for the LP You Rambling Boys of Pleasure: Traditional Ballads and Songs from Ulster (Topic, 1975). He again dealt with music in many of his articles for Dúiche Neill, such as ‘Who Wrote “An Chúileann”? Was it a Benburb Man, Muiris Ó Dúgáin?’ (1988), ‘A Tyrone Woman and Her Songs: Mrs Eileen Keaney of Glenelly and Belfast’ (1992), ‘Dr George Sigerson and Those “Mountains High”’ (1994), ‘The Tunes We Played Going Away’ (2008), and ‘David Hammond (1982–2008): Some Personal Memories’ (2009–10).

In 1990 Proinsias generously donated a large collection of music materials – over 500 items, with indexes – to the Irish Traditional Music Archive: sound recordings, books, journals, programmes, newspapers, etc. They remain in the Archive for the benefit of the public and are part of his legacy. He also served on the ITMA Board.