| Competition: | Irish First Division |
| Winners: | Crusaders |
| Promoted: | Crusaders Donegal Celtic |
| Relegated: | Ballyclare Comrades Ballymoney United |
| Matches: | 132 |
| Total Goals: | 381 |
| Prevseason: | 2004–05 |
| Nextseason: | 2006–07 |
The 2005–06 Irish First Division was the eleventh season of second-tier football in Northern Ireland under the league system at the time. The 2005–06 First Division consisted of 12 clubs.
Crusaders were the champions and were promoted to the 2006–07 Irish Premier League. Donegal Celtic finished second and entered the promotion play-off, defeating Institute 3–1 on aggregate and gaining promotion. Ballyclare Comrades and Ballymoney United were relegated to the Irish Second Division.[1]