1. Yes, but this is not difficult. It is simply a form to be completed.
2. Your solicitor will advise you, but most of the relationships you mention are in the class B threshold, meaning they can each take roughly 30000 from you before paying tax. That is the threshold at the moment but it will be whatever the threshold is when you die. So it could go up ( or down). After that tax is paid at 33% at the moment and again that rate is subject to change.
3.Yes, a simple trust will do this. There are some tax implications but again, solicitor will talk you through it.
Second 3. You can have one or more executors. You can name whoever you want, worst case the named executor can appoint someone in Ireland to take out the grant on their behalf.
4.You can have as much detail as you want. Bring in a list if you like. It is your will, you can do whatever you want in it. But if there are lots of little details it will help to have it in writing.
5. Yes, you can have funeral arrangements in your will, quite common.
If you die leaving Irish assets, your relatives will be subject to irish inheritance tax on them. Otherwise, if the assets are abroad, it will depend on your domicile/residence and of the beneficiaries. Again your solicitor will talk you through all this.