It's the clash of the titans everyone is desperate to see. Ferris versus O'Brien. Amidst the pitch battle, theirs is the ultimate war zone.
If this were a boxing bout, who knows what depths would be plunged to promote it. Mercifully, there is little need for this duo to act like goading guttersnipes. They are best of friends off the field. For months on end they share a room together. But when they cross the white line in opposition for the first time in four years, they will be best of enemies.
Any fighting talk is superfluous. Just light the blue touch paper and retreat upon hearing the first trill of Nigel Owens' whistle at 5.0 this Saturday.
In rugby, the individual battles are key. Each player tries their upmost to beat his opposite number, confident that if his team-mates can follow his example, the spoils will be theirs.
Predicting who will emerge victorious when these two meet is virtually impossible.
"Irresistible force meets immovable object, isn't it?" suggests Mike Ross.
Even more intriguingly, despite their lengthy time together in Ireland camp and their starring roles in, for example, the World Cup win against Australia, they have rarely faced each other in provincial combat.
An hour during Leinster's gritty Christmas 2008 21-13 win in Ravenhill is the only time the duo have opposed each other in competitive fare at senior level.
And that night, it was all about Rocky Elsom as Michael Cheika's side notched another significant scalp en route to their debut Heineken Cup win; at that time Ferris and Ulster's Matt Williams were going nowhere slowly, while few had mapped the teenaged Tullow tyro.
"We played twice against each other at U-21 level as well," recalls O'Brien hazily. Ferris does not want to be reminded. "A couple of times," he grimaces. "Aye, U-19s or something like that. They absolutely smashed us that day. Down in Donnybrook."
Suggest to O'Brien that their unique friendship, not to mention his 100pc record, may reduce the battle to the proverbial piece of cake and he chuckles loudly.
"No, he's not a piece of cake to play against! Fez is in serious form this year. He has been playing well all season. He is a handful to play on the field and I can assure you of that.
"We room together on Ireland trips and I get on very well with him. We would be good friends and be in contact outside of the Ireland camp so we know a fair bit about each other."
That familiarity breeds not contempt, but respect. Declan Kidney spotted the similarities when he first pitched them together as room-mates ahead of the 2010 Six Nations.
The pair share similar upbringings -- Ferris paved the roads in his late teens; O'Brien still mucks around on the family farm. Neither man evolved through the sheltered security of the predominantly cliquish, cloistered schools system.
"Yeah, we kind of emerged in rugby in a different way," agrees Ferris. "We haven't really talked about it that much but I suppose our personalities are alike. It's a natural thing, maybe. We don't have to talk about it.
"It's probably something to do with the way we've been brought up. We had similar upbringings which again reflects in our personalities.
"Maybe Declan saw something in that when he decided to put us together, I don't know. We play in the same position as well, which might have been important to him."
When their international campaigns are at their most intense -- like the eight-week periods of World Cups and Six Nations -- Ferris and O'Brien will share bedroom space more than most married couples. "I'd hate to know what goes on in there," laughs Ross.
They know each other's personal foibles as intimately as they do their professional habits.
"We'd always be sharing notes," says Ferris, "especially when you're away from home for a couple of months at a time. You'd obviously talk about games and different players.
"We get on so well. We more or less talk about everything. He's a good room-mate, who doesn't snore which makes it even better."
There is a chance that, should one of them receive a familiar forearm to the solar plexus this Saturday, there will be a resigned, wounded shrug from the temporarily wounded victim.
"If anything, we give each other compliments more than anything else," adds Ferris. "We know what each of us can do. Sean's a fantastic player. He wasn't European Player of the Year last season for nothing.
"Come next Saturday, me and a lot of other players are going to have to tackle him. He gets on so much ball and we need to be strong to ensure he doesn't make us look stupid."
"Generally you end up nullifying each other," suggests Ross. Still, Leinster will be equally keen to target Ferris, as Joe Schmidt acknowledges.
"He makes a big impact in big matches -- because they're the only ones he plays!" grins Schmidt. "They save him, wrap him in cotton wool and he comes back to play massive games.
"He's a complete player. He has a good offloading game, he attacks players, but knows when to pass, although more often than not he just makes a hole in defenders. He's also good in the line-out and adds some steel to the mauls."
This season, Ferris is easily the form player of the two; his inclusion as one of five nominees for the ERC's Player of the Year award yesterday confirms as much. Ironically, O'Brien was last year's winner.
"At this stage though, Seany knows me inside out and I know him inside out. Obviously you want to do better than your opposite number. If you do that, you know you're going to have a good day at the office," said Ferris.
"I'm sure it will be no different for Sean, he'll want to outgun me. It's a team performance but usually if you beat your opposite man, you have a feeling it's going to be a good day."
So who wins when the irresistible force and immovable object collide? Ross has a degree in biotechnology and even he doesn't know. "I'll leave you to decide that."
Better still. Wait for Saturday.
Date of Birth: Aug 2, 1985 (age 26)
Birthplace: Portadown, Co Armagh
Height: 1.93m (6' 4")
Weight: 112kg (17st 8lb)
Ireland caps: 35 (4 WC; 14 Six Nations)
Debut: November 2006 v Pacific Islands
Ulster caps/tries: 95/12 (League 66/10; ERC 29/2)
Nickname: Tullow Tank
Date of Birth: Feb 14, 1987 (age 25)
Birthplace: Tullow, Co Carlow
Height: 1.88m (6' 2")
Weight: 108kg (16st 13lb)
Ireland caps: 19 (4 WC; 10 Six Nations)
Debut: November 2009 v Fiji
Leinster caps/tries: 70/13
(League 44 /7; ERC 26/6)