The following is a list of the IRMAs number-one singles of 1999. The dates shown are Sundays.
| Issue Date | Song | Artist | Ref. | ||
|---|---|---|---|---|---|
| 3 January | align=center rowspan=4 | "Chocolate Salty Balls" | align=center rowspan=4 | Chef | |
| 10 January | |||||
| 17 January | |||||
| 24 January | |||||
| 31 January | align=center rowspan=3 | "Pretty Fly (For a White Guy)" | align=center rowspan=3 | The Offspring | |
| 7 February | |||||
| 14 February | |||||
| 21 February | align=center rowspan=6 | "...Baby One More Time" | align=center rowspan=6 | Britney Spears | [1] |
| 28 February | |||||
| 7 March | |||||
| 14 March | |||||
| 21 March | |||||
| 28 March | |||||
| 4 April | align=center rowspan=5 | "Swear It Again" | align=center rowspan=5 | Westlife | |
| 11 April | |||||
| 18 April | |||||
| 25 April | |||||
| 2 May | |||||
| 9 May | align=center rowspan=2 | "No Scrubs" | align=center rowspan=2 | TLC | |
| 16 May | |||||
| 23 May | align=center rowspan=2 | "Candle for Kosovo" | align=center rowspan=2 | Rain / Various Artists | |
| 30 May | |||||
| 6 June | align=center rowspan=2 | "Everybody's Free (To Wear Sunscreen)" | align=center rowspan=2 | Baz Luhrmann | |
| 13 June | |||||
| 20 June | "That Don't Impress Me Much" | Shania Twain | |||
| 27 June | align=center rowspan=4 | "9pm (Till I Come)" | align=center rowspan=4 | ATB | |
| 4 July | |||||
| 11 July | |||||
| 18 July | |||||
| 25 July | "Livin' La Vida Loca" | Ricky Martin | |||
| 1 August | align=center rowspan=2 | "When You Say Nothing At All" | align=center rowspan=2 | Ronan Keating | |
| 8 August | |||||
| 15 August | align=center rowspan=2 | "If I Let You Go" | align=center rowspan=2 | Westlife | |
| 22 August | |||||
| 29 August | align=center rowspan=3 | "Mambo No. 5" | align=center rowspan=3 | Lou Bega | |
| 5 September | |||||
| 12 September | |||||
| 19 September | align=center rowspan=5 | "Blue (Da Ba Dee)" | align=center rowspan=5 | Eiffel 65 | |
| 26 September | |||||
| 3 October | |||||
| 10 October | |||||
| 17 October | |||||
| 24 October | align=center rowspan=4 | "Flying Without Wings" | align=center rowspan=4 | Westlife | |
| 31 October | |||||
| 7 November | |||||
| 14 November | |||||
| 21 November | align=center rowspan=2 | "32 Counties" | align=center rowspan=2 | Dustin | |
| 28 November | |||||
| 5 December | "Everyday I Love You" | Boyzone | |||
| 12 December | "I Try" | Macy Gray | |||
| 19 December | align=center rowspan=2 | "I Have a Dream"/"Seasons In The Sun" | align=center rowspan=2 | Westlife | |
| 26 December |