Yes they are. It averages at about 15°C/59°F. You won't get freezing temperatures so there would not be any problems travelling. Ireland's weather is unpredictable, but September is not a bad month to travel in Ireland. You will be likely to get rain at some point, but no extreme cold or extreme heat.
About 15°C, but it can be a lot higher sometimes. Check the site below for more details on Ireland's weather and climate.
That would depend on where in the world you are measuring or travelling from, and if you are travelling, it would depend on what way you were travelling.
Passport
None, you just go over the border.
That may depend on what you regard as warm. It averages about 15°C/59°F, but it can be a lot higher sometimes. Ireland's weather is unpredictable so you can't be sure of what way it will be.
Not if they're travelling from Northern Ireland, but from anywhere else he/she does.
No. There is a common travel agreement between Ireland and Britain. You will not need a passport at any time when travelling across the sea between Britain and Ireland. Neither do you need one when travelling in Ireland, from the south of the country to the North. There is no border on the island. There is open travel between the islands/countries of Ireland and Britain. Google 'Common Travel Area Britain Ireland' .
There is no simple answer to that. It will depend on a lot of factors, like when you are coming, for how long, where you are staying, what you want to do, where in Ireland you want to go, how are you travelling to and around Ireland.
No, you do not need any shots before travelling to Ireland.
Funderland is a travelling theme park, with strong connections to Ireland.
That will depend on where you are travelling to and from in each country, and the method you are travelling. Flying, you could be in Ireland in about 15 minutes, at their nearest points. By boat it can be done in less than an hour. For some parts, these journeys could take a lot longer.
Countries on or close to the equator have higher temperatures because they are nearer to the sun. Ireland is 53°North, so a long way from the equator and as a result it gets lower temperatures.