The rioters: Gardai believe much of the rioting witnessed in Dublin on Saturday was driven by football hooligans who congregated in the city-centre intent on taking advantage of more formal protests being held against the "Love Ulster" parade.
One line of inquiry being explored by gardaí is that many Dublin-based hooligans who support Glasgow Celtic Football Club congregated in a pub in the O'Connell Street area which is popular with Celtic fans. The pub broadcasts all Celtic's fixtures.
As the disturbances orchestrated by suspected dissident republicans broke out, a large group was seen coming from the pub wearing Celtic jerseys even though there was no Celtic game on Saturday.
Many of the rioters later pictured in the media were wearing Celtic jerseys.
Security sources said that while there was no history of soccer hooliganism in the Republic on the scale of Saturday's riots, a relatively small group of Dublin-based soccer supporters had been involved in mini-riots in recent years and would be "quite capable" of causing a major disturbance.
One senior Garda source said that because the disturbances seemed to have been at least in part organised by soccer hooligans, gardaí would have found it almost impossible to detect the plans in advance. The same source said that those involved could simply have rung and texted each other to arrange a meeting in pubs around O'Connell Street, and a small number could have started and sustained much of the trouble witnessed.
Last week, the Shamrock Rovers Ultras website for fans was visited by a number of people who posted inflammatory anti-unionist comments. Some of these predicted serious violent disturbances on O'Connell Street.
Other postings claimed that those marching in the "Love Ulster" parade would be carrying photographs of at least one man linked to the bombings in Dublin on May 17th 1974, which claimed the lives of 34 people.
One visitor to the site commented: "I'd be astonished if this event passes off peacefully."
Many of those who visited the site, including a poster claiming to be from Sinn Féin, repeatedly appealed for calm over the weekend.
A spokesman for Shamrock Rovers said that the club condemned the violence, which was in no way linked to it. He said that any person could visit a noticeboard under a false name and leave postings. The spokesman said that the Ultras fan club had always condemned violence.
Saturday's riots were not the first time that gardaí have been confronted with serious civil unrest on O'Connell Street linked to drunken Celtic supporters. In May 2001, some 29 people were arrested for public order and criminal damage offences after Celtic fans who had been drinking in pubs around O'Connell Street came on to the streets to celebrate their victory against Glasgow rivals Rangers in a match televised earlier that day.
Last May, Linfield supporters clashed with a group of rival supporters wearing Shamrock Rovers and Celtic jerseys before the Setanta Cup Final in Tolka Park, Drumcondra, Dublin.
There were disturbances last May and June at three games in Dublin involving fans from Bohemians FC, Shelbourne FC and Shamrock Rovers FC. A League of Ireland committee set up to investigate those incidents concluded: "Militant elements of teenagers and young males who align themselves to National League teams view matches, especially those of a traditional crunch nature, between rival clubs as a vehicle for violent behaviour, hooliganism and general misconduct."
Web postings on SRFC Ultras: before and after riot
• "Remember, they marched past the Falls Road, symbolising the number five for the five people the UVF slaughtered in the bookies a few weeks previous. They'll be symbolising the Dublin bombings from the 70s if they're allowed to march Saturday."
• "You can be sure that Republicans of all shades will be out for this one . . . Are we expected to welcome the British queen as well, because this weekend is a feeler for Lizzie's visit? Anyone intent on protesting against loyalists has my 100 per cent backing."
• "So maybe 50 loyalists will need hospital treatment afterwards. Like I say to my kids - don't play with the traffic".
• "There WILL NOT be any peaceful protest at this. Anyone I know that's going to protest is going to bash heads."
• "If there's any flags, placards supporting loyalist paramilitaries . . . beat them back on to their buses, that's if they haven't been set on fire."
• "The kids out rioting were exactly the same as ones who would cause trouble at a Rovers game."