Staying in Schengen

Here is what I have gleaned from reading responses to this question in the past:

1. Good for you for being aware of the limits and wanting to follow them.
2. Count carefully. Days that you enter or exit still count as a day of your stay. For example you leave on a Tuesday for Ireland and come back the following Tuesday. That will only count as 6 days out of the zone, not 7.
3. Cutting it as close as 89 might be putting you at risk. A flight delay or cancelation could cause issues for you. There have been reports of large fines for overstays and then limits on future travels.

Croatia and the Irish Republic are in Schengen.

I question Ton's statement. the Republic of Ireland is definitely not in the Schengen Area. It has a Passport Union (open border with no border controls) with the UK, which is not in Schengen either. Keeping this border "open" is one of the issues of Brexit.
AFAIK no date has yet been set for Croatia joining the Schengen Area.

As to the original post, beware that the limit is "90 days in any 180", so you must not at any time have more than 90 days in the last 180 (including arrival and departure days) in the Schengen Area. You need to re-count every day.
And 89 days is a bit close if there is a risk of delay.
Be sure you know where all the arrival and departure stamps are in your passport in case it is queried.

You need to knew who is in and who is not in the Schengen zone. It is not the same as the EU. And remember it is a look back from each day. For example - today is Jan 23, if I count back 180 days from today and including today, have I been in the Schengen zone for more than 90 days? Tomorrow is a new set of 180 days looking back. I personally would not cut it that close. I would have a pad of at least five days. The downside risk is very high if caught.

With the caveats expressed by the others (your bolt holes are non-Schengen--see page 2), you'll be OK. But there's a risk in cutting it that close. In my experience, the longer the trip (and the older you are...), the more likely you are to get sick somewhere along the way. I was unlucky in 2015 to have the illness hit me not long before I was due to cross the border from Slovenia (Schengen) to Croatia. It was a gastrointestinal thing that prevented my going far from my hotel room. I was lucky to be well enough to travel on Day 89 or Day 90.

Crossing my fingers, I planned a Day 89 run out of France last year, by ferry to the Channel Islands (UK so non-Schengen). I made it. But darned if my onward ferry to the mainland wasn't canceled for 3 days running.

Clearly I am snakebit but lucky. I hope you are equally lucky.

https://www.schengenvisainfo.com/schengen-visa-countries-list/

I agree that 89 days is cutting it close. You are not allowing for any problems.

Regarding Schengen, the following countries are not part of the agreement:

Albania
Bulgaria (awaiting admittace)
Croatia (awaiting admittance)
Cypress
Ireland
Romania (awaiting admittance)
United Kingdom

Please keep in mind that overstaying your stay in Schengen can mean a hefty fine and possible ban for up to 10 years.

Yep, do not push it, certainly in a three month trip you can plan more time in non-Schengen countries.

Also, especially if you choose Croatia (ferry or bus to get there), but for any country, keep a paper trail handy (boarding passes, hotel receipts, etc.) when leaving Italy to come home, if they just see your entry and see it is greater than 90 days, they will start asking questions; be prepared to lay it out clearly for them. You may not have all passport stamps, or all that are legible, your path may be in their computer, but the first check is visual.