Normally blessed with a chipper, upbeat disposition (at least on the outside), the Taoiseach has seemed exceptionally grumpy of late.

Whether it's having a pop at hospital consultants, griping about new TDs having the temerity to challenge him, complaining about the negative media, bemoaning the length of time he had to spend down at Dublin Castle or sitting stony-faced through the recent Dail confidence motion, Ahern is starting to resemble the famously neurotic Peanuts cartoon character Charlie Brown, who often had a black cloud over his head.

Certainly Ahern's default facial expression at the moment is screaming: 'Why does it always rain on me?'

These should be sunny days for the Taoiseach. Three successive electoral victories; a still-strong economy with the highest rate of immigration in the world; peace in the North . . . all extraordinary achievements . . . it points to a place in the pantheon of great taoisigh.

So why the long face?

The obvious answer is, of course, the ongoing investigation by the Mahon tribunal.

Although for many the tribunal's work requires no justification, there is little doubt that Ahern is particularly sore at what he sees as the unwarranted trawl through his personal finances.

In his view, he was called into the tribunal to answer allegations about getting money from Owen O'Callaghan and, even though there isn't a shred of evidence that this ever happened, he is now under investigation for something different.

"He is feeling remarkably sorry for himself. Like all people, the higher he gets, the thinner the skin and he is spending far too much time thinking, " said one close observer. "He is putting on the wounded animal. He feels hurt that he has done such a great job and all he's getting is the Spanish Inquisition."

This source believes that Ahern finds the tribunal process particularly frustrating because, even for the ultimate political strategist, there is basically nothing the Taoiseach can do about it. "It also captures the grey area in his life . . . that thing where the left hand doesn't know what the right hand is doing. That's his way of doing business and it's being held up to the daylight [by the tribunal], " the source said.

There has been some joking in political circles that what is really killing the Taoiseach is having to repay the money he got in loans from friends but, more seriously, the feeling is that Ahern is concerned about his legacy.

The walls of his constituency office at St Luke's are adorned with photographs of Ahern with various world leaders, and those who know him say the Taoiseach is acutely conscious of his place in history and is worried that what is happening at Dublin Castle may damage that.

"He is wondering: 'Am I going to get the legacy I deserve?'" one source said.

His critics would no doubt suggest he will.

Nor is Ahern likely to be thrilled at the prospect that the end of what has been a brilliant political career is fast approaching.

While the belief is that, regardless of what happens at the Mahon tribunal, he will go at a time of his own choosing, the question within Fianna Fail is now whether it will be before or after the local elections.

Inevitably, that brings a certain undermining of his authority.

"It's a creeping kind of thing, but he is so sharp, he can realise it before anybody, and he's probably beginning to sense it a little. It's what happened with Blair.

"You still have authority but people are looking behind you wondering 'Who's coming next?' and you don't have the same clout, " one source said.

Publicly, the party has been rock solid behind him in relation to the tribunal's investigations, but privately, there is some muttering among TDs.

Ahern didn't get where he is today by being unaware of such disquiet.

There have also been suggestions that the Taoiseach is a tad curmudgeonly about the credit that Brian Cowen, his chosen successor, is receiving within Fianna Fail for his role in the election victory. However, this is played down by those who have observed the two men.

While they are certainly not close, the two work well together and, though it will be treated with scepticism in some quarters, the word is that Ahern is quite open in his praise of his finance minister's role in last May's election success.

Tiredness is also pretty much ruled out as a possible source of the Taoiseach's crankiness. "He is not tired.

He looks really well and he feels really well, " said one close observer.

Of course, it should be pointed out that some of the targets of the Taoiseach's ire in recent weeks were totally justifiable.

Most people would agree with Ahern on the consultants . . . it is getting more than a little wearing listening to consultants passionately bemoaning the state of the health service while at the same time their representative body is refusing to agree a deal on new contracts so crucial to reforming the health system.

It was probably also fair enough that Ahern took exception to brand-new Fine Gael TDs having a swipe at him . . .there is an argument that you have to earn the right to slag off the Taoiseach of the day in the Dail.

And while journalists mightn't like it, many people would share Ahern's distaste for the media pack in full flow (although the Taoiseach's antipathy towards the media does seem to have deepened in recent times).

But there is little doubt that the workings of the Mahon tribunal are what lie at the heart of his irritation. And the problem for Bertie 'Charlie Brown' Ahern is that the cloud that is Mahon isn't going anywhere anytime soon.