TONY BOYLE has a theory he doesn't want to see proven any time soon, but which he believes offers challengers to Donegal's standing as the country's top football force the best chance of unseating the champions.
"I can't see anyone beating Donegal unless they score goals. Jim (McGuinness) has a very smart system going and, so far, it's working great. I suppose the question is what happens on a day when Donegal fall five, six, seven points behind?
"I'm sure there's a Plan B to cope with that but will it work? They won't know until it's put to the test and it won't be put to the test until it has to. That's most likely to happen when the opposition score goals," said Boyle.
As one of Donegal's best forwards in a career which extended from 1990 to 2001, featuring 107 appearances, Boyle is well qualified to comment on the impact of goals on games.
"It's going to be very hard for any opposition to open up a decent lead against Donegal through points only. That's why they need goals, but, to date, nobody has been able to get them.
"The thing is, though, that however good a defensive system may be, the day always comes when things go wrong and you concede say, two goals. Then it's a new situation altogether," he said.
It hasn't happened to Donegal in 14 championship games (see panel) in the McGuinness era. They've conceded only four goals in those games, two against Cavan in separate seasons and one each against Kerry and Cork last year.
The Cork goal came two minutes into stoppage time in the second half of the All-Ireland semi-final, at which stage Donegal were five points clear and safely through to the final.
A 14-4 goal differential in 14 games partially explains why Donegal have done so well over the past two years, but defensive solidity has been equally significant. They have conceded an average of 10.6 points per game, ranging from a high of 0-14 to a low of 0-8.
Any squad that can keep their average giveaway to fewer than 11 points will always win a lot more games than it loses, as has been the case with Donegal, who have won 13 of their last 14 championship games.
Inevitably, though, the day will come when the goal drought ends or when some opposition kicks a string of early points to open up a sizeable lead. What then for Donegal?
"If it's to happen, it's most likely it will be through goals. The best defences in the world in any sport have a day when they are hit for more scores than usual. It will happen to Donegal too.
"Someone will open up a decent lead some day and that's when we'll see how Plan B works. I have no do doubt Jim has a Plan B, and probably C and D as well, but you can never practise them because the situation hasn't arisen. You have to hope they kick in when the occasion demands," said Boyle.
Apart from being so defensively secure, a feature of Donegal's run has been the manner in which they establish the agenda in the first half. They have been ahead at half-time in 10 of 14 championship games in the McGuinness era, trailed in three and were level in one.
Tyrone (2011 and 2012) and Kildare (2011) led them, but were beaten.
Usually, the pattern is for Donegal to lead at half-time and while it may be by only a small margin, it has proven enough, except for the 2011 clash with Dublin, who recovered from a two-point interval deficit to win by two points in the lowest scoring All-Ireland semi-final of the 70-minute era which started in 1975.
"It's as if teams have it in their heads that Donegal don't fade at any stage in the second half – which, in fairness, they don't. So, if a team is behind at half-time, they find it very hard to get back in the second half," Boyle said.
If that mindset prevails, it makes it all the more important for the opposition to take the initiative in the first half by jolting Donegal back and presenting them with challenges they have not been encountering.
Tyrone managed it in the 2011 Ulster semi-final at a time when the McGuinness project was in its very early stages. Donegal trailed by five points after 25 minutes, but pared three points off the deficit before half-time and went on to win by three points.
"Donegal should have been trailing by around 10 points, not five after 25 minutes. Tyrone missed a lot of chances in the first half hour and Donegal came back at them very strongly.
"Donegal did it very well, but other than that day, they haven't had much experience of chasing a decent sized lead," said Boyle.
Down's 2-17 return against Derry in the quarter-final has raised hopes that they could be the team to ask really tough questions of the Donegal defence, even allowing for the 11-point defeat in the Ulster final last year.
"Down were in that game for three-quarters of the way, so it would be wrong to just look at the final score (2-18 to 0-13) and think it was all very easy for Donegal. Down had a few goal chances in the first half, but didn't take them.
"If they did, who knows what might have happened? Down teams are always capable of making goal chances – I'd be very wary of them," said Boyle.
Down's high yield against Derry will encourage them, but Boyle doesn't envisage tomorrow's game being as loose.
"It was a throwback to 20 years ago. I wouldn't expect to see Down get anything like as many chances tomorrow."
For all that, it's worth recalling that Tyrone created a lot of openings against Donegal in the opening 50 minutes (the wides count was 11-1 in their favour) of the quarter-final, but ultimately paid the price for their inaccuracy.
"We were well beaten around midfield for long stages and we're going into Sunday's game without big Neil (Gallagher), which is a concern. The other worry would be that after focussing on the Tyrone game since the draw was made last October, it might be hard to bring the same intensity to this one.
"The players are listening to everyone saying that after beating Tyrone, winning Ulster is a formality. Jim and the lads won't be thinking that way, but I'd be concerned over the complacency among supporters. Okay, so they're not playing, but that sort of mood in a county is dangerous," said Boyle.
However, he expects Donegal to stay on track for an Ulster treble but does not believe that defeat would be particularly damaging.
"Bringing back-to-back All-Irelands to Donegal has been Jim's priority from the start of this year. He would also love to win the three-in-a-row in Ulster, but it wouldn't be the end of the world of they had to go the 'back door' route.
"An interesting thing about this Donegal squad is that it has a lot of students and teachers, who are off for the summer, so they have a fair amount of time to rest and recuperate after games. That's a help," said Boyle.
It's 20 years since he was on the Donegal squad that set out in defence of the All-Ireland title, an adventure that ended in the Ulster final when they lost to Derry in a deluge which should have led to the postponement of the game.
Boyle, an All Star from the previous year, missed the campaign after a viral problem with a nerve in his shoulder left him immobilised. His absence was a huge loss to Donegal, especially in the Ulster final when his opportunism might well have won the day.
"It was horrible to miss the defence of the Ulster and All-Ireland titles. Of course there was no 'back door' then, so we were gone after losing the Ulster final. Not that I would have made it back anyway," said Boyle.
He is disappointed by the number of one-sided games outside of Ulster this year and fears that if it continues, it will lead to a drop in interest in the provincial championships in the longer term.
"We're still getting competitive games in Ulster, but elsewhere has been poor. The danger is that people will start thinking that the championship doesn't begin until the August Bank Holiday weekend when eight teams play in the quarter-finals," he said.
For now though, it's all about Donegal's attempt to stay on line to achieve a historic Ulster treble.
Said Boyle: "I'd expect the lads will beat Down, but it's no foregone conclusion. It never is against Down and you must remember we will be without Karl (Lacey) and Neil (Gallagher). They will be big losses."