You say internationalism is something revolutionaries have to practice. But why? I could see from a Trotskyist perspective why this might be the case--if the party itself had the intention of taking power, then internal experience of internationalism would have a 'prefigurative' quality (similar to the way anarchists see horizontal organisation).
And funnily enough, it is something they have never practiced, and probably never will. This is because they don't see it like that. Even the future party, they say will be a federation of different national parties. So what they try to do is try to set up national parties and become their leaders.
But for left-communists I still don't see why local organisations would be less suited to the tasks at hand.
Fundamentally because we think that every communist nucleus has to see the world with the perspective of an international whole to achieve its tasks. A local organization which is not a part of an international centralized body, like all other purely local organizations regardless of the tendency, would be consumed by the day to day activities and events going on in that locality. Because at the end of the day, no matter what it is always the conditions shaping the consciousness. A purely local activity will create a mind-set and eventually politics of its own.
Regarding convincing people of the importance of internationalism by embodying it in the organisation, again, that seems like it would be significant for an organisation that actively recruits, as the Trotskyists do, but I don't see why a local left-communist organisation couldn't convincingly argue for the necessity of an international workers' movement. I see where you're coming from with this point, but it seems like a minor issue.
It is not just about arguing with people or recruiting them though. It is about the stance left communist organizations take within the struggles of the working class as a part of their interventions.
Participating in a workers' struggle not just as a local group of militant proletarians but a group of local militant proletarians who are also part of an international whole actually significantly strengthens the practical intervention.
The extracts you posted are helpful, but they seem to mostly be arguing for the importance of internationalism in the workers' movement itself (which I fully accept), but then arguing from historical precedent when it comes to showing why left-communist organisations must be international.
Actually, the argument is made from a practical point as well:
It's probably not a very practical line of inquiry anyway, since there are no left-communist organisations in Ireland, as far as I know.
Just because there isn't one of course doesn't mean there won't ever be though.