Firstly, you're heading in to a very complicated situation in relation to taxation, particularly because you are moving to Greece which could have separate taxation issues and may or may not have taxation agreements in place with Ireland. I strongly suggest that you seek professional advice before deciding what to do. In the long run it will be more than worth it to you. I'm not a tax expert but I'll have a go at some of your queries:
1. This is not the case. You may only have one principal private residence. If you move to Greece and continue to live in your house when you come back one week in 4 and you do not rent it out you will not pay CGT when you sell. The 3 year rule relates to residency and income tax not CGT.
2. If you rent it out you are liable to income tax on the rent less any interest that you pay on the mortgage, less any expenses you have in relation to the property. This will be included in your overall income and here is where you need professional advice as to how that relates to your income from Greece and where you are paid from etc.
3. Tax Relief at Source (mortgage interest relief) will no longer be available to you once it ceases to be your principal private residence. As to if one week in 4 counts as it being your PPR I don't know.
4. You did not pay stamp duty on it 18 months ago. If you move out and rent it out within 5 years of owning it, the stamp duty that you would have paid as an investor when you bought it becomes payable from the day you rent it out.
That just scratches the surface of what you need to consider. Perhaps your employer would pay for you to seek proper advice. You'll never get a full and clear answer to your queries on a site like this. They're too complex.