The album features a selection of music that she recorded for the soundtrack to The Celts, a BBC documentary series about the origins, growth, and influence of Celtic culture.

Following her work on the soundtrack to the romantic comedy film The Frog Prince (1985), Enya's first major project as a solo artist followed when, in 1985, she was invited by producer Tony McAuley to compose a track for his 1987 BBC television documentary series The Celts.

As a coincidence, Enya had recorded a track named "March of the Celts" before she was asked to be involved, and decided to submit it to the project.

[5] "Boadicea", which means "victorious", is a reference to the queen Boudica of the British-Celtic Iceni tribe in East Anglia who led a resistance against the occupying forces of the Roman Empire in 60 A.D., but was defeated and subsequently poisoned herself.

[5] As a song about her was already written, Richardson wished for a new track that depicted the idea of "being spellbound" by Boudica, which turned into "I Want Tomorrow".

[5] In the liner notes of the album's 1992 reissue, "I Want Tomorrow" is described, simply, as "thoughts of the present" and "March of the Celts" "echoes from the past".

[9] "Deireadh an Tuath", which translates from Irish as "End of the Tribe", refers to past spirits and the fertile soil that helps ensure the future of the Celtic people, which is celebrated in the annual Gaelic festival Samhain, held on 31 October.

[9] "The Sun in the Stream" was inspired by the legend of the Salmon of Knowledge, a creature written about in various texts in Irish mythology who "possessed all the truth in the world".

Following the album's reissue in 1992, "The Celts" was released as a single on 2 November 1992[16] with "Eclipse", a previously unreleased track from the Enya sessions, as a B-side.

Another unreleased track, "Spaghetti Western Theme from The Celts", was released in 2005 as a B-side for Enya's 2005 single "Amarantine".

Filmmaker David Bickley reused music from the soundtrack in The Memory of Earth,[17] an instalment in his documentary trilogy Mythological Lands.

[9] The album was reissued on 16 November 1992 as The Celts by WEA[18] in Europe and Reprise Records in the United States.

[9] In 2009, The Celts had a limited Japanese reissue on Super High Material CD with "Eclipse" added as a bonus track.

"Boadicea" was also sampled on the answer song to "I Don't Wanna Know", "You Should Really Know" by the Pirates featuring Shola Ama, Naila Boss and Ishani, which peaked at number eight in the UK in 2004.

"Boadicea" with "Ready or Not" was also sampled by R&B group Nina Sky on their single "Time to Go" featuring rapper Angie Martinez, from the mixtape presented by Cipha Sounds.

In 2011, a small sample of "Boadicea" was used on "Der erste Winter" by German singer Cassandra Steen for the album Mir so nah.

On Nigerian-American singer Rotimi's 2019 album The Beauty of Becoming, he sampled the song on a track entitled "In My Bed", which also featured the rapper Wale.

In 2022, "I Don't Wanna Know" was covered in "Creepin'" by American record producer Metro Boomin, Canadian singer the Weeknd, and Atlanta-based rapper 21 Savage, and by proxy Enya is credited as a writer.

On 22 July 2016, Bosnian-Swedish DJ Salvatore Ganacci released the single "Dive", which heavily samples "Boadicea" and features vocals from Alex Aris.

He compared the sound of the album to Clannad following their shift in musical style in the early 1980s, "echoing, shimmering vocals and instrumentals".

The lyrics to "St. Patrick" adapted from the Irish hymn "Deus Meus, Adiuva Me" by Mael Ísu Ua Brolcháin, though it is credited as "Traditional".