| Countries: |  Ireland |
| Champions: | Ulster |
| Count: | 1 |
| Runnersup: | Leinster |
| Matches: | 110 |
| Top Point Scorer: | Felipe Contepomi (Leinster) (287 points) |
| Top Try Scorer: | Tommy Bowe (Ulster) Jason Forster (Newport Gwent Dragons) (10 tries) |
| Website: | www.rabodirectpro12.com |
| Prevseason: | 2004–05 |
| Nextseason: | 2006–07 |
The 2005–06 Celtic League was the fifth Celtic League season, and the third following the introduction of regional rugby in Wales. Eleven teams, from three countries participated. The final table was dominated by the Irish, with the top three spots taken by Ulster, Leinster and Munster respectively. The 2005–06 season was the first (and last) season for which the "free weekend" was awarded points for. Due to the odd number of teams participating, for every round of fixtures, one team has a so-called "free weekend". Each time this occurred, the team was awarded four points, so although at the end of the season it did not alter the positions (each team has a guaranteed 8 points), the table is skewed during the season according to the fixtures.
| Team | Stadium | Capacity | City, Area | |
|---|---|---|---|---|
| Border Reivers | 6,000 | Galashiels, Scotland | ||
| Cardiff Blues | Cardiff Arms Park | 12,500 | Cardiff, Wales | |
| 5,500 | Galway, Republic of Ireland | |||
| Edinburgh Gunners | 67,800 | Edinburgh, Scotland | ||
| Glasgow Warriors | Firhill Stadium Hughenden Stadium | 10,887 6,000 | Glasgow, Scotland | |
| Donnybrook Stadium | 6,500 | Dublin, Republic of Ireland | ||
| Llanelli Scarlets | Stradey Park | 10,800 | Llanelli, Wales | |
| Thomond Park Musgrave Park | 13,200 8,300 | Limerick, Republic of Ireland Cork, Republic of Ireland | ||
| Newport Gwent Dragons | 12,000 | Newport, Wales | ||
| Ospreys | 20,500 | Swansea, Wales | ||
| 12,800 | Belfast, Northern Ireland |
| Pos | Team | Pld | W | D | L | PF | PA | PD | TF | TA | TBP | LBP | Pts | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | Ulster | 20 | 15 | 1 | 4 | 510 | 347 | +163 | 49 | 31 | 3 | 2 | 75 | |
| 2 | Leinster | 20 | 14 | 0 | 6 | 545 | 427 | +118 | 59 | 45 | 8 | 2 | 74 | |
| 3 | Munster | 20 | 12 | 0 | 8 | 439 | 372 | +67 | 49 | 42 | 7 | 3 | 66 | |
| 4 | Cardiff Blues | 20 | 11 | 0 | 9 | 475 | 389 | +86 | 51 | 38 | 6 | 5 | 63 | |
| 5 | Edinburgh Gunners | 20 | 11 | 0 | 9 | 418 | 415 | +3 | 48 | 45 | 5 | 3 | 60 | |
| 6 | Llanelli Scarlets | 20 | 10 | 1 | 9 | 418 | 402 | +16 | 49 | 37 | 3 | 4 | 57 | |
| 7 | Ospreys | 20 | 11 | 0 | 9 | 381 | 409 | −28 | 33 | 38 | 1 | 2 | 55 | |
| 8 | Newport Gwent Dragons | 20 | 7 | 0 | 13 | 355 | 456 | −101 | 40 | 51 | 2 | 7 | 45 | |
| 9 | Border Reivers | 20 | 7 | 0 | 13 | 386 | 501 | −115 | 39 | 59 | 1 | 7 | 44 | |
| 10 | Connacht | 20 | 6 | 0 | 14 | 325 | 466 | −141 | 28 | 51 | 1 | 4 | 37 | |
| 11 | Glasgow Warriors | 20 | 5 | 0 | 15 | 371 | 439 | −68 | 39 | 47 | 2 | 7 | 37 | |
| Due to the uneven number of participating teams, each team had two free weekends and were awarded 4 match points each time. | ||||||||||||||
| Source: RaboDirect PRO12 | ||||||||||||||
Note: Flags to the left of player names indicate national team as has been defined under IRB eligibility rules, or primary nationality for players who have not yet earned international senior caps. Players may hold one or more non-IRB nationalities.
| Rank | Player | Club | Points[1] | |
|---|---|---|---|---|
| 1 | 287 | |||
| 2 | 229 | |||
| 3 | 173 | |||
| 4 | 125 | |||
| 5 | 122 |
| Rank | Player | Club | Tries[2] | |
|---|---|---|---|---|
| 1 | Tommy Bowe | Ulster | 10 | |
| Jason Forster | Newport Gwent Dragons | |||
| 3 |  Felipe Contepomi | Leinster | 9 | |
| 4 | Nikki Walker | Border Reivers | 8 | |
| 5 | Chris Czekaj | Cardiff Blues | 7 |