This is hardly the first time Gerry Adams’s past has erupted into uncomfortable questions and unwelcome headlines for Sinn Féin.

But few controversies have so sharply illustrated the contradictions of his role and the difficulties in reconciling the party’s past with its present-day ambitions.

The moral compromises necessary for peace sit uncomfortably with the requirements of modern politics. And the answers that Adams has always given no longer satisfy many.

Adams is the link between the two sides of Sinn Féin - the political wing of the IRA during the Northern conflict, and the left-wing anti-austerity party of southern politics.

Indeed, he has planned and overseen the transition from one to the other; he is the embodiment of his party and its recent history in a way that none of the other leaders even come close to.

He is also the personification of the link between the Northern and Southern wings of the party, and keeping the two together is of paramount importance for it.

But precisely because of his success in transforming Sinn Féin into a mainstream political force, he is now increasingly being held to the standards of the political mainstream.

As the last week has demonstrated, they are standards he cannot meet.

Cannot tell all

Gerry Adams simply cannot tell all that he knows about the IRA and its deeds.

But his failure to do so - at least in the case of the Stack family this week - is politically untenable.

He occupies the space between a rock and a hard place, and as long as he stays leader, Sinn Féin will be squeezed.

The legacy of darker aspects of the Republican culture of silence and loyalty has dogged Adams in recent years.

In April 2014, he was arrested and detained for questioning about the murder of Jean McConville in 1972.

Later that year, Belfast woman Mairía Cahill revealed that sexual abuse of her had been privately investigated and covered up by the republican movement, and the following year, Louth man Paudie McGahon made similar claims.

All these issues aroused a storm of uncomfortable publicity and criticism for Adams. Looking back at them now, though, it seems Sinn Féin suffered little political damage.

However, a range of opinion polls conducted in the wake of these controversies paints a slightly different picture. They suggest Sinn Féin suffered some short-term damage in the immediate aftermath of highly negative publicity, but recovered soon afterwards.

The real damage may have been not among Sinn Féin supporters, but among those who might support them in the future. The controversies may reinforce negative perceptions that prevent growth.

At the last general election, Sinn Féin gained 9 seats and saw its share of the vote rise to just under 14 per cent.

However, the haul of seats masked a disappointing election for the party, which had polled much higher over the previous years and entertained hopes of more than 30 seats.

It confirmed a pattern of Sinn Féin underperforming its poll ratings on election day.

The formation of the Government also outflanked the party. It had ruled out coalition in a bid to push Fianna Fáil and Fine Gael into government together, leaving Sinn Féin to dominate the opposition.

No accident

Micheál Martin’s unwillingness to enter government scotched that plan. It is no accident that Fianna Fáil has been the target of many of Sinn Féin’s attacks since then: The antipathy is returned in spades.

Many people will not consider voting for Sinn Féin in any circumstances and this has led to a sort of ceiling on Sinn Féin’s vote.

Adams is not the sole reason for this, but he is the symbol of it. That ceiling is much higher than it used to be, but it is still a serious impediment to the party’s growth and, therefore, its ambition to win power.

Much of this is linked to the party’s association with the IRA and the bloody years of conflict in the North.

It’s true that for many younger voters, this stuff is a history lesson, not a political debate. Consequently, Sinn Féin’s supporters are much more likely to be younger.

In this week's Irish Times/Ipsos poll, for example, Sinn Féin's support among likely voters in the 18-24 group was at 27 per cent; among the over-50s, it was 12 per cent.

The corollary of this is that the party’s past links to paramilitary violence continue to be a drag on its support. And those links are personified in its leader.

Circle the wagons

The first thing that Sinn Féin does when Adams is confronted by a controversy from his past is to circle the wagons and defend him.

The party discipline that by turns impresses and alarms their opponents engages, and nobody in a position of prominence or leadership will break ranks to criticise the leader. It was the same this week.

It’s not true that there’s no discussion about the issue or about the wider issue of Adams’s leadership of the party throughout its ranks - it’s just that these things are kept in the family.

Nobody will challenge the leadership line in public. That is either testament to the commitment and loyalty of activists, or slightly creepy, depending on your point of view.

But like Enda Kenny’s future in Fine Gael, the subject of Adams’s continued leadership of the party will not go away. He is 68 now, and has served for 33 years as party leader, a preposterous length of time in conventional politics.

There is no doubt that Adams is the towering figure in the republican movement - the man who brought its members from war to peace.

But he will not go on forever. At some point, probably in the next couple of years, their futures will diverge.

Observers and opponents have been predicting his retirement for a long time, his allies point out. But sooner or later they will be right.

Replacing him while keeping the Northern and Southern wings of the party together remains the biggest strategic challenge for Sinn Féin.