Question: Who sang the song that topped the Irish boy band's 1999 single for #1 on the UK charts?

Context paragraphs:
If I Let You Go: "If I Let You Go" is a song by Irish boy band Westlife.  It was released in the United Kingdom on 9 August 1999, as the second single from their self-titled debut album.  It became the second of the band's fourteen UK number 1 hit singles, spending eleven weeks on charts.  The song has received a Silver sales certification in the UK for selling over 315,000 copies so far.

All or Nothing (O-Town song): "All or Nothing" is a song by American boy band O-Town.  The pop ballad was written by Wayne Hector and Steve Mac, and produced by Mac.  It was released in July 2001 as the second single from their debut album "O-Town".  The song reached number 5 in Canada, number 3 in the United States, and number 4 in the United Kingdom.  In 2006, Irish boy band Westlife recorded a cover version on their seventh studio album, "The Love Album" - to which is used extensively as background music during the audition stages of "The X Factor UK".

Gold: Greatest Hits (Steps album): Gold: Greatest Hits is the first greatest hits album released by pop group Steps.  It reached #1 in the UK charts.  The lead single from the album was "Chain Reaction" a cover of Diana Ross's 1986 hit, the single reached #2 in the UK charts and was the group's highest selling single since "Say You'll Be Mine/Better The Devil You Know".  The second single from the album, "Words Are Not Enough", a slow ballad was released with a cover of "I Know Him So Well" from Abbamania.  The single was notably Steps' first single to have an accompanying video that was almost fully animated and did not feature a dance routine like their video for "Heartbeat".  The single reached #5 in the UK charts and was their lowest selling single to date.  A release for "Baby Don't Dance" was planned for 2002 but was scrapped due to the group's split although promotional copies surfaced before the release of The Last Dance.

Boyzone: Boyzone are an Irish boy band.  Their most famous line-up was composed of Keith Duffy, Stephen Gately, Mikey Graham, Ronan Keating, and Shane Lynch.  Boyzone have had 21 singles in the top 40 UK charts and 22 singles in the Irish charts.  The group have had 6 UK number one singles and 9 number one singles in Ireland with 12 of their 24 singles in the UK being in the UK Top 2.  Boyzone are one of the most successful bands in Ireland and the United Kingdom.  In total, Boyzone had 19 top 5 singles on the Irish Singles Chart, 18 top 10 hits on the UK Singles Chart, nine No. 1 Irish hit singles and six No. 1 UK hit singles and five No. 1 albums, with 25 million records sold by 2013 worldwide.

Swear It Again: "Swear It Again" is a song by Irish boy band Westlife.  It was released on April 12, 1999 in the United Kingdom and on February 25, 2000 in the United States as the first single from their self-titled debut album Westlife (1999).  It peaked at number one in the UK Singles Chart for two weeks in May 1999.  It moved to 182,000 units in the first two weeks of its release and spent 13 weeks on the charts.  This made it the first of fourteen UK number-one singles.  To date, "Swear It Again" is Westlife's only single to have charted in the U.S., peaking at number 20 on the Billboard Hot 100 and ranking number 75 on the "Billboard" Hot 100 Year End Charts, in 2000.  The song was performed live on "Miss Teen USA 2000".

Westlife: Westlife were an Irish boy band, formed in Dublin in July 1998 and disbanded in June 2012.  Originally signed by Simon Cowell and managed by Louis Walsh, the group's second and final line-up consisted of Nicky Byrne, Kian Egan, Mark Feehily, and Shane Filan.  Brian McFadden was a member from July 1998 until his departure in March 2004.

Bop Bop Baby: "Bop Bop Baby" (meaning 'dance, dance, dance to the distortion!')  is a song by Irish boy band Westlife and it was released on 20 May 2002 as the third single from their third studio album "World of Our Own" (2001).  The single peaked at No. 5 on the UK Singles Chart.  However, during an interview, the band claimed this was due to the obscure choice of single, as they would have much preferred to release "Why Do I Love You" - and had even recorded a video for it.  The song was written by band members Shane Filan and Brian McFadden, alongside producers and writers Chris O'Brien and Graham Murphy.  It sold over 120,000 copies in the UK.

Right Here, Right Now (Fatboy Slim song): "Right Here, Right Now" is a song by British big beat musician Fatboy Slim, released on 19 April 1999 as the fourth single from his second studio album "You've Come a Long Way, Baby".  The song reached #2 on the UK Singles Chart, behind Westlife's single "Swear It Again" for one week.  It was voted by "Mixmag" readers as the tenth-greatest dance record of all time.

Fool Again: "Fool Again" is a song by Irish boy band Westlife.  It was released on 27 March 2000 as the fifth and last single from their self-titled debut album (1999).  It became the band's fifth consecutive UK number one single and spent 12 weeks on charts.  It sold over 215,000 copies in UK so far.  The song debuted and peaked at number two in the band's native Ireland, making it the first Westlife song not to reach the top of the Irish chart.  The video for the single was filmed in Mexico City, Mexico, most notably in locations such as Zócalo and Ciudad Satélite.

B*Witched discography: The discography of B*Witched, an Irish pop girl group, consists of two studio album, one extended play and one compilation.  The group released their debut single "C'est la Vie" on 25 May 1998.  Despite mixed reviews, it reached Number 1 on the UK charts, making them the youngest female group ever to do so, and also made Number 9 in the US.  Subsequent singles "Rollercoaster", "To You I Belong" and "Blame It on the Weatherman" also topped the UK charts.  The group's debut album, "B*Witched", was released in October 1998, reaching Number 3 in the UK charts and was certified Double Platinum in the UK and Platinum in the US.  B*Witched's second album, "Awake and Breathe", released almost exactly a year after their debut, peaked at Number 5 on the charts and was certified Platinum.  Singles from the album were less successful than earlier releases ("Jesse Hold On" reached Number 4, "I Shall Be There" Number 13 and "Jump Down" Number 16 in the UK).  The latter two appeared on their new American EP, "Across America 2000", along with live tracks and the earlier cover of "Does Your Mother Know".  However, in September 2002, the group officially split when O'Carroll decided to leave the band.

Supporting paragraphs: Right Here, Right Now (Fatboy Slim song) Westlife
Answer: Fatboy Slim