Three months in the EU then six in the CTA.

It's important to keep in mind that you can stay for 90 days in the Schengen zone, not 3 months.

For such a long stay in GB and Ireland, you may be asked questions at immigration control. They may want to know what you will be doing, how you will pay for it and that you aren't intending to stay permanently. Or looking for a job. Answer wrong or arose their suspicion and you may be denied entry. But that would be rare and unusual.

And as long as you don't exceed the 90 days within each 180 day period, you will have no issues in the Schengen zone.

Paul,

"What should I expect at costumes entering and exiting the CTA?"

I'm assuming you mean customs (aka Immigration & Passport control).

As the previous reply mentioned, there could be questions on whether you have adequate financial resources. When you enter the CTA, the stamp on your Passport will state "does not provide leave to seek employment"). They may also want some assurance that you have adequate medical coverage.

You can only stay in the Republic of Ireland for a maximum of 90 days, notwithstanding the 6 months entry that can be permitted into the UK. In both cases the actual length granted would be at the discretion of the officer concerned.

Immigration typically ask questions to make sure that you are there as a tourist and not for a reason that requires a visa (i.e. to live or work there). Such a long stay will likely cause them to ask a few questions.

Some of the questions that I have been asked when entering the UK on vacation ...
1) How long will you be staying? (routinely)
2) Where are you staying?
3) What is the purpose of your visit? (routinely)
4) Have you been to the UK before? When?
5) Can I see your return ticket? (Happened to me when I was in my 20s and backpacking but not recently)
6) How will you pay for your return ticket (when I told them I hadn't bought a return ticket yet)? Can I see your credit card? (after telling them I would pay with card)
7) Do you have an friends who live in the U.K.? Are you visiting them?

When I actually legally lived in the U.K. with a valid work/residence visa, I got a ton of questions every time. They were making sure that I was still in compliance with the terms of my visa.

Not long ago we had a question from someone who had spent a full 90 day in Schengen and then went to the UK. They wanted to then travel by train through but not stopping in Schengen countries. All posters agreed that would not be allowed.

So my advice is to leave yourself a cushion of a few days. Only spend 85 days in Schengen so that you can go back briefly if you need to.

I was going to leave the same advice as Karen. Give yourself a few extra days (I like her suggestion of staying 85 days, having 5 leftover). This will help if you want to connect or take a train through a country, or if you meet some friends in England who want to take a day or weekend trip to Paris. You don't know what will come up - best to have some extra room to allow for other trips into the Schengen zone.

Thanks everyone. I like the reassurance that I'm doing the right thing. I can't wait to get exploring.