The 2004–05 Irish First Division was the tenth season of second-tier football in Northern Ireland under the league system at the time.
Armagh City were the champions and were promoted to the 2005–06 Irish Premier League.
Glenavon finished second and entered the promotion play-off, defeating Crusaders 3–2 on aggregate and gaining promotion.
Ballinamallard United were relegated to the Irish Second Division.
Ballymoney United were also due to be relegated, but were given a reprieve due to Omagh Town (who were relegated from the Premier League) going out of business.