ROI and Southern Ireland are one an the same!!
It has 26 counties the other 6 are occupied by the UK ! and it referred to as Northern Ireland or the North of Ireland.
26 counties of Southern Ireland or the Irish Republic are:
Carlow, Cavan, Clare, Cork, Donegal, Dublin, Galway, Kerry, Kildare, Kilkenny, Leitrim, Laois, Limerick, Louth, Longford, Mayo, Meath, Monaghan, Offaly, Roscommon, Sligo, Tipperary, Waterford, Wicklow, Wexford, Westmeath.
These counties are indicated on the registration plate of a vehicle, by the letter, ie, DL for Donegal, G for Galway
ie 03-WH-1233, would indicate that the truck is yr 2003 and was registered in WestMeath
You dont need a passport to go by Boat. You do if you are flying by Ryan Air or an ID card, its their T&Cs.