The British government published an October 2015 report on the state of Northern Ireland paramilitaries, drafted by MI5 and the Police Service of Northern Ireland (PSNI). The short of it is that all terrorist groups which were active during the Troubles still exist. Except for a few dissident republicans, all other terrorists are dedicated to peace and achieving their ideological objectives via politics and community activism.
Though each group's organisation and leadership is still active, their leadership has significantly reduced command and control capability. Even if they wanted to, these organisations would not be able to return to peak strength observed during the Troubles.
Loyalist terrorism has largely degenerated into organised crime, with some elements, like the Loyalist Volunteer Force (LVF), dedicated to little else. Consequently, loyalist terrorists continue to recruit and sometimes acquire new weapons. As stated these assets are then deployed mostly for criminal enterprise.
A minority of dissident republicans have clustered around old and new iterations of the IRA, such as Continuity IRA, Real IRA / New IRA, and disaffected Irish National Liberation Army (INLA) members. NIRA have claimed recently (April 2019) that Brexit has helped them recruit new volunteers. Though this is problematic, it is important to remember that they are a minority force which operates without widespread community support.
The Provisional IRA, the largest republican organisation by far, is dedicated to peace. Officially, they are not the same as their political wing: Sinn Fein. But the reality is that both the PIRA and Sinn Fein are commanded by the Provisional Army Council (PAC). Gerry Adams, one of Sinn Fein's most important leaders, famously claims he was never in the IRA. It is a claim few believe, and more recently PIRA veterans have said on the record that the claim is a blatant lie.
While Sinn Fein and the PIRA may be slowly drifting apart with time, during the Troubles, and still for the most part, there is little if any distinction between the two. Sinn Fein's leadership was populated with men who gained authority through PIRA activity and thus sat on the Provisional Army Council.
For these reasons, there is minimal possibility of a return to violence on the scale we have seen in the past. There is also no difference between the opinion of the PIRA and Sinn Fein, as they are both guided by the same command structure via the PAC. They are two sides of the same coin.
Regarding 'IRA' naming conventions...
It is also worth noting that "official sinn fein" is ambiguous. In 1969 there was a split in the IRA between the Official IRA and Provisional IRA. Official IRA was represented by Official Sinn Fein. However, the officials eventually decided to focus on politics, and their members who disagreed created the INLA. Official Sinn Fein therefore became the Workers' Party of Ireland.
Over the last century there have been multiple iterations of the IRA. These are not just different names for the same entity. Some are historical organisations which are now defunct, while others are contemporary. The 'Provisional' IRA split from the 'Official' IRA in 1969, and then the 'Continuity' IRA split from the PIRA in 1986, followed by the 'Real' IRA who split from the PIRA in 1997. The 'New' IRA is the latest variant, and is a cluster of various physical force republicans who began working together only a few years ago.
Support for these organisations varies, but since their inception the PIRA has dominated Republican politics and terrorism. Those groups who split from the PIRA, and what became of the OIRA, have never been as popular as the PIRA. The NIRA is an especially small group, even if it has been recruiting recently.
Critically: each IRA iteration is for a united Ireland, and consequently is against anything which threatens to divide the island of Ireland. And Brexit certainly carries that risk.