1. Dear *****, the first thing is that your own researches have been fruitful and are correct. You can apply for Irish citizenship by descent based upon your grandparent being born on the island of Ireland (North or South). Here is a link to the webpages of the Irish Naturalisation & Immigration Service which explains how you apply for Irish citizenship based on your grandparent being born in Ireland http://www.inis.gov.ie/en/inis/pages/wp11000024. You will also be able to find the application forms referenced on this website as it is the authority you apply to for obtaining citizenship in Ireland.
2. I would not advise you to get professional advice. Simply apply yourself. The greatest virtue you will need is patience as the Irish authorities can be quite slow. So allow about 9 months for the process. If your application lacks anything, the Irish authorities will request it from you. The Irish authorities don't have the approach of the UK Visas & Immigration which tend to reject applications as defective if something is missing. So do it yourself.
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