1) If the chargeable gain is under the exemption limit, no liability arises and therefore no need pay tax during the year. If a liability arises, depending on the date of disposal payments should be made with a CGT payslip by either 15 December in the tax year for disposals between 1 January and 30 November, and by 31 January of the following year for disposals in December
2) A CGT return should be made after the end of the year (on your tax return if completing one) or on a CGT1 if no liability to income tax by 31 October of the following year
3) Comversion rate would be the rate pertaining at the date of the acquirisition and disposal, using daily rates
4) Dividends received by you are not relevant to CGT as they are liable to income tax, however, if dividends were reinvested, then they are treated as acquisitions for the purpose fo CGT
5) As a UK non-resident you are liable to UK tax on the disposal of UK situs assets, and UK shares would fall under this. However, the UK CGT exemption is very generous compared to the Irish exemption and if the gain is small, there is likely to be no UK CGT payable.
Please note that the calculation of CGT in Ireland and the UK are different in respect of shareholdings, and you need to prepare the calculation for each country based on that country's CGT rules.