1. Dear *****e, at the outset, it is perfectly lawful under both Irish and UK law to hold a second passport and to be a national of another second country, other than Ireland or the UK. Accordingly, it is perfectly lawful for you to hold an Irish passport as well as a British passport and to use each one as appropriate. For example, you can in future use the Irish passport for travelling in EU countries where freedom of movement is recognised and also use the UK PASSPORT in Commonwealth countries where the UK has special rights for its passport holders.
2. Secondly, you are correct in that entitlement to an irish passport is based on having either an Irish parent or an irish born grandparent. Here, if your grandparent was born anywhere on the island of Ireland, including Northern Ireland, you can apply for an Irish passport based on their birth in Ireland.
3. It is necessary to take out full Irish nationality in order to obtain an Irish passport. There is no such thing as a half way house where you don't have full Irish citizenship. YOu are either an Irish citizen with an Irish passport or you are not.
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