Every year there are dozens of street protests in Dublin's city centre. They tend to focus on a small number of streets/areas in the city centre (Kildare St, College Green, O'Connell St etc). These can cause significant traffic disruption, indeed as such that is part of their intent (to get attention)
Why not designate a large area in Phoenix Park for protests ? An added bonus is it would make it easier for the Garda to police & count protesters.