Would you call the IRA (Irish Republican Army) an idea, or would you have called it a group?

Sure - after all, the original IRA was a single group. Over time, that group split into various different organizations - most prominently in 1969 - each of which operated under the banner of the IRA, but had separate organizational names. While there were different groups that claimed that title, "IRA" always referred to the group(s) itself - not the thing they were fighting for, which was "Irish Republicanism."

Analogizing that to the current topic, "Antifa" is more equivalent to "Irish Republicanism" - the belief system that motivates a number of organizations, but not the organization itself. Just like it would have been nonsensical to say that "Irish Republicanism gave a speech to supporters earlier this afternoon," it's not accurate to refer to Antifa as if it were an organization, rather than collection of ideas or beliefs.

Since "Antifa" is also a system of tactics, it's also akin to saying something like, "In Northern Ireland today, Terrorism placed a number of explosive devices near a local office." Again, it's nonsensical - Terrorism was a tactic of the IRA, but wasn't an organization or a group of people.