My sister is moving to NY for a few months for work.
The extended irish family (me, irishfella, irishbaby, my parents, my other sister) are planning a visit in October, for about 7 days.
So…what should we do, bearing in mind we have a very small person, but that grandparents are ok with a night or two of babysitting duty.
Is she moving to New York City, or another part of New York state? It’s a large state, with areas that are very, very different from each other so knowing the exact area will help.
Manhattan. Not sure where exactly, but we’re fine with using the Subway to get about.
A carrier backpack or ?Snuggli? (the chest holder things) is probably better than a stroller, in general. Most of the subway stations do not have elevators, so you would have to drag both it & child up and down stairs – and there isn’t enough room for a stroller during rush hours anyway. If you need one, a lightweight foldup model is better than the stroller/assault tanks I see around here. 
Do 15 month old children have interests? There are a number of nice playgrounds in Manhattan. Union Square (14th st) and Madison Square (23rd St.) both have nice playgrounds. Central Park, obviously, has playgrounds.
Through Aug 12, there are children’s music performances in Madison Sq. Park twice a week
https://www.madisonsquarepark.org/programs/madsqkids.aspx
This site recommends 10 Toddler-friendly activities:
http://gonyc.about.com/od/familyfun/tp/tot_attractions.htm
Dylan’s Candy Bar is every child’s dream…even the stairs are made of candy!
Peanut Butter & Co will be wonderful for everyone, no matter their age, because it is full of deliciousness.
Central Park Zoo This is especially good for small kids because it is a tiny zoo. You can see the whole thing in about an hour so your kid isn’t exhausted at the end of the day.
There is no reason not to take her to the Met or MOMA with you. Kids love the colors and love to find the animals and people and boats in the paintings and sculptures. My kids grew up in Manhattan and we were there all of the time. There is also a nice small zoo in Central Park that I recommend as well. People and truck watching as well are always fun with little kids.
http://www.circleline42.com/new-york-cruises.aspx
Cirlce Line tour of NYC. Boat that goes around the island of Manhattan. Folding chairs for adults to sit, room enough to bring a kid in a stroller. Nice break on a nice day as you pass by the major buildings on the perimeter of NYC. See Ellis Island, South Street Seaport, etc. Good close-up view of The Statue of Liberty. Or take the cruise up the Hudson River to Bear Mountain.
Are all 6 of you, planning on staying at your sister’s place? Hope it’s big enough.
And there’s a petting zoo. You can feed some cute looking goats, llama type creatures, and even a bull. (I was too scared to feed the bull.)
Isn’t it a cow? Not a bull?
I wasn’t sure. I can’t remember if it had horns. Do cows have horns? Though I think I remember the horns looked like they’d been filed down or something.
Either way, the last time I went, I was too scared to feed it. And I’m in my twenties and this was last year. Go ahead. Deride me for being the city slicker that I am.
The goats were quite sweet, though. Well, the nanny goat was. She came right up to the cage and ate. The other goats wouldn’t. Too good for the likes of pellets, they were!
I think it’s a cow. I dunno how good the idea of a bull in the middle of tons children is. The goats are cute if needy. It’s all pet me, scratch me, feed me. Gets a tad tiresome. I like the sheep, they’re above it all or maybe they’re just really dumb.
To the OP, if you decide to do the Central Park Zoo, not too far is the Central Park carousel which is also fun for kids.