Irritated with over helpful new colleague

She's obviously wanting to make a good impression. Just explain, (nicely!) that's not how things are done. You're grateful for her help but always best to ask first. Good luck!

I would feel annoyed, too. Is she after your job?

Too many cooks spoil the broth! Let her down gently and explain the established routines. She’s only trying to make a good impression which is to be applauded but a scatter gun approach to making arrangements is the road to misunderstandings.
Bide your time. She’ll pick up the rules soon enough and will soon settle into it.

Well I think I'd start by applauding her initiative, then suggest she holds back a little until her induction is complete and explain why that's important, then finish by telling her that her helpfulness has been noticed.

I have to do this kind of thing all the time when new volunteers are over-eager. I don't want to quash their helpfulness, but I do want it to be directed in ways that don't make extra work for me!

"I notice you offered to help find a new date. Thanks for showing initiative and offering but we operate on the principle that the person trying to arrange a meeting is the only one who gets involved. We do this to avoid any potential confusion. I am really pleased to see that you are able to use your initiative though, and once your induction is completed will definitely be making use of that!"

Madgran77 is spot on.

Oh dear poor woman trying to be helpful and show initiative and not appreciated. I've been in her shoes with people wanting things ' the way we do it round here' and 'we've always done it like that'. Its uncomfortable being the 'new girl' cut her some slack and be glad she is a willing worker.