The inaugural United Rugby Championship gets under way tonight and becomes the tenth iteration of what began as the Celtic League back in 2001.

Previous versions of the championship had different names, different teams, varying number of participants and formats, most recently it was the Guinness Pro14.

The opening weekend sees perennial champions Leinster host one of the new boys of the tournament, South Africa's Vodacom Bulls, while Munster’s clash with the Cell C Sharks is live on RTÉ.

It’s all a far cry from very first league game when Bridgend entertained Pontypridd in the Brewery Field.

Twenty years ago, six years after the game went professional, the four Irish provinces, Edinburgh Reivers and Glasgow from Scotland, and Wales’ nine top teams: Bridgend, Caerphilly, Cardiff, Ebbw Vale, Llanelli, Neath, Newport, Pontypridd and Swansea squared off in a new-look tournament.

Leinster celebrate with the Celtic League trophy

The 15 sides were divided into two pools and the four Irish sides occupied the top two spots in their respective groups, with Leinster, coached by Matt Williams, going on to beat Munster 24-20 in a memorable final in Lansdowne Road.

"In the early days of professionalism, in England, the likes of Leicester, Wasps and Northampton became fully professional from day one," URC tournament director of the last 17 years David Jordan tells RTÉ Sport.

"There was a realisation that this sort of part-time / professional set-up which existed in the other countries wasn't going to wash because you are going up against really professional sides in Europe.

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"That meant that Scotland, Wales and Ireland needed to look at the issue collectively and that was basically where the idea was originally formed.

"In the case of the Welsh they went from a club-based system to regions.

"That was more like a Celtic Cup where you had Welsh and Scottish clubs playing a tournament (from 1999 to 2001) that led to the beginnings of the Celtic League but eventually it was realised that we needed to have full-time professional rugby and that was based on a regional provincial-type model.

"In Scotland we went to two teams, Glasgow and Edinburgh. The Irish had the four provinces and the five Welsh regions.

"In time we had the 10 teams playing what was the Celtic League.

"The fundamental raison d’etre for the competition was to raise standards, to create a professional environment to allow clubs and players to succeed at club, provincial and international level.

"That was our primary goal from a rugby point of view but it's also a commercial game so we now have a full commercial team and broadcasters etc., where revenue is a major part of what we are about now, as well as creating an entertaining product, a dynamic product."

Munster players celebrate after the Celtic League final in 2003

The only change to the second season was the addition of Scottish Borders, while Munster beat Neath in that year’s final.

Ahead of the start of the 2003/04 season, the Welsh clubs amalgamated and formed the Cardiff Blues, Celtic Warriors, Neath–Swansea Ospreys, Newport Gwent Dragons and Llanelli Scarlets.

The 2003 RWC and the Six Nations in the spring meant that the clubs played for most of the season minus their international stars. Llanelli Scarlets finished ahead of Ulster in the table after the competition was re-formatted to a straight league structure.

By the start of the 2004/05 season, the Celtic Warriors had been disbanded and the title was won by the Ospreys. That year the IRFU decided they would use the league standings to determine which sides would represent Ireland in the Heineken Cup.

Moves by the Welsh clubs to take part in an Anglo-Welsh competition disrupted the 2005/06 pre-season but when that was sorted out the tournament saw increased attendances. Ulster, courtesy of a late David Humphreys drop-goal against Ospreys, won the league.

Ulster celebrate with the Celtic Cup

Renamed the Magners League from 2006/07, Ospreys claimed the spoils. The highlight from an Irish perspective was a Leinster v Ulster clash at Lansdowne Road that attracted almost 50,000 people in the last game played at the Dublin 4 arena before its redevelopment.

Leinster won their second title in 2007/08 with just 10 teams competing, Scottish Borders having been disbanded ahead of the season.

Munster claimed their second title the following year, losing only four games.

The competition returned to a league and play-off format in 2009/10 with Leinster, who beat Munster in the semi-final, losing the final 17-12 to the Ospreys, for whom Tommy Bowe scored a crucial try.

Italy entered the fray in 2010/11 with Aironi and Benetton Treviso joining up.

"When Italy became competitive in the Six Nations it was important that their clubs were part of a professional organisation," said Jordan.

The final that season came down to another all-Irish clash with Munster beating newly crowned Heineken Cup champs Leinster in Thomond Park.

Munster and Leinster in the 2011 final

From 2011/12 until 2014 the competition became known as the RaboDirect Pro12 with Ospreys snatching the title off Leinster with a late flourish in the RDS in the final.

Aironi bowed out for the 2012/13 season to be replaced by Zebre with Ulster, who topped the table in the regular season, losing to Leinster in another final held in the RDS.

Leinster were again victorious in the 2013/14 final, this time accounting for Glasgow 34-12 in the RDS. The game marked the last appearance of Brian O’Driscoll after a stellar career for club, country and Lions.

Guinness came on board as the title sponsor for the 2014/15 season with Leinster, who had become the first club to retain the title the year before, failing to make the play-offs. Glasgow beat Munster in the final.

The 2015/16 season was all about Connacht as the western province won their first major silverware of the professional era. They beat Glasgow in the semi-final before a memorable victory over Leinster in Murrayfield.

Connacht captain John Muldoon lifts the trophy

The shocks continued the following year when Scarlets became the first side to win an away semi-final, blasting past Leinster at the RDS. More was to come when the Welsh side obliterated Munster at the Aviva, Tadhg Beirne among the try-scorers.

The arrival of the Southern Kings and the Cheetahs from South Africa for the 2017/18 season saw the 14 teams split into two conferences, with Leinster beating Scarlets in an Aviva finale.

It was around this time that chief executive Martin Anayi suggested that it was a matter of "when, not if" a team from the USA would join the tournament, while Germany was also mentioned in dispatches.

Tournament director David Jordan

"We did get approached by a number of territories, including the US, wanting to be part of our competition," continued Jordan.

"We have welcomed in clubs from other countries whereas England and France are a closed shop. But it has to work for us, it has to be competitive and be commercially viable.

"At the end of the day there’s a number of factors. There was interest from that direction but we have to look at it from what’s right for the competition and that it adds to the standard."

While the Cheetahs finished third in their conference in their first season, the Kings failed to make any impression, eventually winning just four games across three seasons.

League of its own? New format faces familiar questions

Leinster would go on to win the next three titles in a row, beating Glasgow in the final in 2019, before accounting for Ulster in the next year's decider.

That finale came after a mid-season suspension due to the coronavirus pandemic, which meant that only 15 rounds of 21 were played.

When games did restart it was decided to play a round of derbies to finalise the standings instead of forcing teams to travel abroad.

The 2020/21 season was again played behind closed doors due to public health guideline sand reverted to a Pro12 format with the Southern Kings entering liquidation and the Cheetahs not permitted to travel because of the pandemic.

It was the same story, however, with no team able to derail Leo Cullen’s kingpins, Munster again coming up short in the RDS final.

The shortened format led to the once-off Rainbow Cup, which was initially to include the new South African teams playing some rounds in Europe.

However, the logistical issues surrounding getting the new franchises into Europe meant that it was restructured so that the winners of a European-only competition and a South Africa-only competition met in a final.

Benetton topped the northern hemisphere standings and went on to claim their first major silverware by beating the Bulls in the final, which was held in Italy.

Which brings us back to this season and the introduction of South Africa's finest: Bulls, Sharks, Lions and Stormers, having forsaken Super Rugby, are in with the in-crowd now.

Leinster have been the dominant force, famously using more than 50 players in a season to win some of the last four titles.

It's great for fans of the province but that dominance does leave an awful lot of questions about the quality of the tournament when a team can win while using players that wouldn't have made the Probables or Possibles teams of yesteryear.

On the challenges facing the latest version of the much rejigged tournament, Jordan, a former CEO of Glasgow Warriors, said: "It's about raising standards. Leinster have won our competition now for a number of years.

"We need to be more competitive from top to bottom. It's been quite difficult to increase the number of teams while working in the same window.

"Our format this year is to avoid international games, to get clubs playing with their internationals more often.

"It is about increasing the competitiveness of all our clubs and hopefully that will lead to better performances in Europe and better performances on the international field as well.

"We think the different players with different approaches to the game will help."

Follow Munster v Sharks, Saturday 25 September, 7.35pm, via our live blog on RTÉ.ie/sport and the RTÉ News app or watch live on RTÉ2 and RTÉ Player