Although the album displays the band’s traditional Celtic sound, it features techniques and styles unusual to the genre, such as occasional double-tracking recording and occasional instances of instruments that differ from Celtic music, such as cello, electric bass and flügelhorn, leading Allmusic to say the album "yields a sound that is unique to the group and yet clearly in touch with tradition".

"[8] The eager, growing reputation of the band lead to them recording their debut album, Lúnasa, later in the year before its release on 16 January 1998.

"[9] The success of their debut album and their touring led to the band signed a three-album contract to American independent Celtic music label Green Linnet Records in 1999, a label described by Lúnasa biographer Bran San Martin as "then one of the premier outlets for modern Celtic music.

[9] Nonetheless, although McGoldrick and McSherry had left the band, they returned as guest musicians for the new album, sharing pipe duties.

"[9] The band underwent "a great deal of experimentation" as they searched for a way in which to "utilize the studio to focus their energies and bring out varying facets off their sound.

"[11] The sessions incorporated techniques highly atypical to usual studio-recorded traditional music, such as multitrack recording, used often in instances of harmonization.

[1] The band's approach to recording the album included multiple instances of double-tracking certain instruments, which lead one critic said "yields a sound that is unique to the group and yet clearly in touch with tradition.

"[3] Mike Sutton of Musical Traditions, explaining the track's contrasts, said "the fiddle-pipes duet on 'The Floating Crowbar' [gives] way to a dazzling whistle lead (well supported by guitar, bass and bodhran) on 'McGlinchey's', before the pipes and fiddle return to enrich the mixture on 'The Almost Reel'.

[1] “The Miller of Drohan" is a slow reel which displays "a more ruminative, delicate side of their music-making, especially through Hutchinson’s playing of cello and bowed bass.

"[3] Elsewhere on the album, the traditional "Lafferty's" featuring "bass, fiddle, flute, guitar, and pipes [building] a rumbling momentum as if," according to one critic, "determined to drag Irish folkmusic kicking and screaming into the 21st century,"[13] whilst "Stolen Apples" features a horn arrangement by Patrick Fitzpatrick.

[20] The album cover of Otherworld, designed by Naoimh Ingram,[10] was described by one critic as "an arty photo of light glancing off rippling water—the band's name in blurry cursive script—a mystical title.

"[4] The band ultimately toured worldwide in promotion of the album, "and even played to a sold-out crowd at the Hollywood Bowl in the summer of 2000.

"[1] Geoffrey Himes of The Washington Post was favourable, calling it "impressive" and highlighting Hutchinson's contributions as key, saying "his vigorous bowing and plucking make the underlying rhythms in these jigs and reels more muscular and obvious than ever before.

"[22] Tony Montague of The Georgia Straight said "Otherworld is as close to perfection as any recording of instrumental Celtic music I can recall, and there aren’t enough fresh superlatives to praise it adequately," calling the band's musicianship "flawless" and the tunes "excellent," adding "the arrangements are impressively innovative.

"[12] Also favourable in their opinion were Roots World who commented that "the band's attraction lies in the tight, inventive improvisation, the wide repertoire and the excellent playing of the four regulars.

"[16] Mike Sutton of Musical Traditions called the album "the genuine article": "The sound balance and arrangements may be studio-smooth, rather than bar-room rough, but these are musicians who know their stuff.

"[15] Alex Monaghan of The Living Tradition was similarly favourable, saying that the album is "so full of power and passion that it is almost out of this world" and concluding that "highlights are impossible to pick, it's all consistently wonderful.

With a bigger budget, they were able to achieve greater production values, which went a long way to capturing the power of their live performances, with "The Butlers of Glen Avenue/Sliabh Russell/Cathal McConnell's" being a true standout.

[26] On the band's "best of" album The Story So Far (2008), three tracks from Otherworld are included: "The Miller of Drohan", "The Floating Crowbar" and "O'Carolan's Welcome/Rolling in the Barrel.