Notwithstanding any provision of the Corporation Tax Acts, income -
(a) arising to Nítrigin Éireann Teoranta in any accounting period ending in the period commencing on the 1st day of January, 1987, and ending on the 31st day of December, 1999, from the business of supplying gas purchased from Bord Gáis Éireann to Irish Fertilizer Industries Limited under a contract between Nítrigin Éireann Teoranta and Irish Fertilizer Industries Limited, and
(b) which but for this section would have been chargeable to corporation tax under Case I of Schedule D,
shall be exempt from corporation tax.