History could be made as two SSE Airtricity Men's Premier Division teams look to qualify for a European competition outright for the first time. Here's all you need to know.
STATE OF PLAY
The summer of European qualifiers comes to an end on Thursday night.
Let's begin with Shelbourne.
The Champions League qualifiers first-round draw took place in mid-June with the original clash of Shels and Linfield on 9 July.
The Drumcondra side won that tie with the Blues but failed to take their next two opportunities in the Champions League and Europa League. Linfield, by contrast, won two Conference League ties to make it to the play-offs.
Last week's first leg could have gone either way. Joey O'Brien's side didn't start well and Linfield were finding their feet before a red card for Matthew Fitzpatrick changed the game.
VAR also made some interventions for penalties, the second of which Harry Wood scored.
Mipo Odubeko made up a for a miss from the spot with a goal from play and then the contest looked over at 2-0, Shels having that man advantage.
But Linfield came roaring back with a Kieran Offord goal and they missed their own spot-kick to equalise. Evan Caffrey restored a two-goal lead to give Shels an outstanding chance of qualification.
It's 3-1 Shelbourne going into a second leg, so we know Linfield will need to score at least twice to take it to extra-time. It's a big ask to stop Shelbourne qualifying for a league phase for the first time.
But Linfield boss David Healy hasn't given up hope, telling the BBC: "Getting the first goal is paramount. If we get it, then who knows what can happen after that. With 11 [players], an excitable crowd, a little bit of electricity in the atmosphere, hopefully we're a little bit more clinical."
AND THE HOOPS?
They've done it the hard way. Drogheda United's expulsion from Europe saw Shamrock Rovers promoted to the second round of the Conference League. They've defeated St Joseph's (Gibraltar) and Ballkani (Kosovo) to get to the play-offs.
Santa Clara of Portugal were next up, a task which many feared but within the Tallaght side was a quiet confidence they could cause a shock.
Having gone 1-0 down, Danny Grant and Danny Mandroiu scored either side of the break to register one of, if not the most, impressive away victory recorded by an Irish side.
A League of Ireland team has never come through three rounds of Europe to qualify for a competition.
Likewise, victory for Shamrock Rovers would be the first time a non-champion has progressed to the league phase.
TV
RTÉ Sport broadcast the home leg of Linfield v Shelbourne last week. Premier Sports have acquired the rights to the return leg in Belfast tonight.
Shamrock Rovers v Santa Clara is available on LOITV for a €10 pay-per-view fee.
ONLINE
You can follow a live blog on both matches on RTÉ.ie/sport and the RTÉ News app.
RICHES AND REALISM
As we stated last week, Shels have already claimed €1.7m in prize money from their Champions League win. Qualification for the league phase of the Conference League would add another €2m.
For a club that has been losing an average of €1m a year under their current model, and is struggling to get back to Europe next year, this is a game-changing scenario. Defeat tonight would be a disaster of epic proportions.
Shamrock Rovers have just under €1m to their name from their run so far. After banking €7m from last year's competition, another progression to the league phase would be significant.
As champions-elect in this year's Premier Division, they will have even more riches coming their way next campaign as they enter the Champions League route.
It has to be demoralising for other clubs, particularly those with little chance of qualifying for Europe, yet there is also a rising tides logic.
Yesterday's academy report highlighting deficiencies came on the same day the FAI also announced they would be cutting staff.
Irish football has enjoyed few good days in the last number of years, yet the once-maligned league is now delivering the majority of its good news stories as well as players and resources for its custodian.
An extra €7m could be headed to these shores tonight, with solidarity money from UEFA stemming from qualification dripping down to other Premier Division sides.
The FAI and clubs have been lobbying the state to provide more resources to grow the game here.
Shamrock Rovers boss Stephen Bradley feels tonight could make a major difference:
"All I know is that we all have a part to play in Irish football to make it better.
"Our part and Shelbourne's part is on the pitch tonight. If we can get both teams into the league phase of a European competition, that can only help the government in trusting Irish football and where it's going and help support it in the areas that we need to support."
BACK TO MATTERS ON THE PITCH
The hope is Shels will be able to repeat their result from Windsor Park or even better it. The 1-1 draw in July was not a convincing Shels display, nor did we see the best of Linfield.
Cliché this may be, but the start and first goal of the match will be telling. It would make it a near-impossible task for the home side if the League of Ireland outfit moved 4-1 ahead.
There has been some key performers in the Reds side, none more so than Harry Wood who continues to deliver in big moments.
Mipo Odubeko caused Linfield lots of problems in the first Windsor Park clash and they'll look to exploit him once more.
It will be interesting to see how Joey O'Brien approaches the tie, with the option of playing on the break through Odubeko supported by John Martin, or could Daniel Kelly and Evan Caffrey start rather than be utilised from the bench? Caffrey, in particular, found great joy last week as he came on at the break.
"It's not about hanging on to what we have or hoping for the best," O'Brien told reporters. "It's about going out and trying to win the game. When you sit back and you think, 'Oh, we're OK here', I think that's when you get punished. I don't want us to be like that."
The approach is also a consideration for the Hoops. Their lead is just a single goal which can be wiped out in an instant.
Stephen Bradley has highlighted "electric pace" in the Portuguese outfit and how that can be dealt with when leading the tie is anyone's guess.
Bradley said: "Santa Clara will have their moments, but we’ll also have ours. So it’s important when we have them, we’re ruthless in those moments."
Rory Gaffney will lead the line and only Connor Malley is out of the squad for the Hoops through injury. Jack Byrne has been left out after a disagreement with his manager.
Around 8,000 fans are expected in Tallaght for one of the biggest nights the venue has seen since opening in 2009.
A dull 0-0 would be enough for the Hoops, but the last seven weeks have shown European nights always provide drama. This could be Irish football's greatest ever one.
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