The Óglaigh na hÉireann white lettering was emblazoned on a red background.
Armed and uniformed gardaí formed a cordon around Little Gerald Griffin Street while mourners called to Cross’s funeral home.
More than 100 people gathered at the house prior to the removal to Corpus Christi church in Moyross.
Mr Campion’s widow Maureen was accompanied by her children Gerard, aged 14, and Lyndsey, aged 12.
The hunt for Mr Campion’s killers continued with house-to-house inquiries in Thomondgate, where he was shot last Thursday.
The two main suspects are believed to be linked to the McCarthy-Dundon gang.
One of them is related to one of the five men convicted of the abduction and murder in January 2003 of crime boss Kieran Keane, carried out by the McCarthy-Dundon gang.
Gardaí suspect that Mr Campion, aged 35, was targeted in revenge for the murder of McCarthy-Dundon gang member, ‘Fat Frankie’ Ryan on September 17 last. Gary Campion, a brother of Noel, has been charged with that killing and, along with a number of others, with the murder in 2002 of nightclub security manager Brian Fitzgerald.
Noel Campion was recently released from a six- month jail sentence for a public order offence. In 1999 he was jailed for nine years for an armed raid on a Limerick oil depot which involved a shootout with gardaí.
There will be a large garda presence for today’s funeral, with armed patrols at potential flash-points in the city.
Meanwhile, city council workers were clearing up the remnants of bonfires lit in Southill and Ballinacurra West by people who took to the streets on Thursday night to celebrate Mr Campion’s murder.