Down south, the tribal justification doesn't wash and it's not as if there was any lack of political alternatives.
If it's a united Ireland you're after, you could work for it through big political parties that form governments.
If it's workers' rights that most concern you, you could help revive Labour.
If socialism or identity politics are what you want, help yourself from the generous buffet on the left.
And if it's the camaraderie of working together in a common purpose, why would you want to get involved with a culture of bullying and intimidation by a shamelessly populist party with no coherent message?
Dr Matt Treacy, who spent 30 years in the IRA, four in Portlaoise Prison, and 10 years in Leinster House as an assistant to Martin Ferris TD, quit the party in 2016, thoroughly disillusioned.
He calculated recently that Sinn Fein, who in 2014 won 264 council seats in elections north and south, have lost more than 10pc of their councillors. In the Republic, 22 have either resigned or been thrown out.
Claims of bullying and intimidation are made public with increasingly regularity.
In April, for instance, Councillor Sorcha O'Neill of Kildare County Council resigned, saying she felt "intimidated", and that though she had raised "multiple concerns" with Sinn Fein HQ over recent years, she had failed to get any more than a "casual response".
Although she loved Sinn Fein, "the party environment in Kildare is a hostile place to work in". Five party activists left with her.
"When you have members coming to you saying they are losing sleep and are distraught, we came to a decision that it was just best to leave the party because it was so unhealthy."
In June, after a bitter internal dispute in Tipperary, a grassroots meeting voted to expel Councillor Seamus Morris from the local party organisation.
According to Morris, some Sinn Fein rank and file couldn't accept attempts to appeal to people beyond the republican base and set out to bully him. There was "a hostile environment", and a "politburo" made up of members of the party in South Tipperary had been spreading slanderous stories that had landed him in hospital suffering from stress.
In June, the former Longford-Westmeath general election candidate Councillor Paul Hogan said bullying was rife within the party and alleged there had been an "unrelenting" campaign against him.
He had documented 80 accusations that included "kangaroo courts, threats, intimidation, bullying, witch-hunts and whispering campaigns", and claimed the party had done nothing about them.
On August 1, three councillors broke their silence over a long-running row in Wicklow which came about because they objected to a leader of their six-strong group and a nominee for the position of chairman of the County Council being imposed on them without consultation.
Despite the rumoured employment as mediator of Sean Hughes from Co Armagh - an IRA grandee nicknamed 'The Surgeon' by security forces - they were stripped of the party whip in March.
Their appeal has now been rejected by the Ard Chomhairle, who said they had refused to take direction from the party.
Sinn Fein favours "robots over human beings", said one of them. "On the issue of toeing the line and obeying the county leadership, [it] is almost unbearable at times, with no leeway allowed."
Nothing to worry about, explained Gerry Adams. The party has "zero tolerance for bullying", and the local disputes are just "growing pains".
Mary Lou McDonald agreed with Gerry.
"You will have all the aches and pains and bumps and bullying in every organisation," but Sinn Fein had all the right "procedures and processes" and was a "fairly harmonious, fairly friendly and increasing outward-looking organisation."
Matt Treacy disputes that, alleging that "the list of Sinn Fein workers, including former elected representatives, who have been paid off in out-of-court settlements to prevent the embarrassment of the workers' friends being exposed for what they are would fill a few pages."
Previously the author of books on the IRA, the Communist Party of Ireland and The Year of the Dubs (about the hurling and football teams of which he is a life-long follower), Treacy had been getting fed up with the party for some time, initially over money.
Like Cork East TD Sandra McLellan, who jumped ship over the requirement that she give what she earned over the average industrial wage to the party ("entirely voluntary donations" according to Sinn Fein), he left in 2016, fed up with being hassled.
He was even more fed up with the favouritism that involved some people having their mortgages, cars and childcare paid for.
"They were being looked after under the table," he wrote in his new book, A Tunnel to the Moon: The End of the Irish Republican Army, which will be launched next month but is already available from the Lulu website.
It's an enjoyable read, not least because Treacy can be very funny as well as revealing about the culture of the republican movement and the splits and anger over the party leadership's insistence on endorsing the politically correct position on major social issues.
"I'd just had enough of them," he wrote to me. "I think the final straw, apart from extortion, was being at a meeting where people were queuing up to call Mairia Cahill a whore and a slut. It was like something from the Cultural Revolution and particularly ironic as it came from people who get highly offended at sexism."
He has himself been subjected to many of what he calls Stalinist tactics. As well as a whispering campaign.
What kind of person would join that party?