This has been asked a few times in the past. If I find the links Ill post them here.
In short if your company relocates they can offer you a job at the new location and if you do not wish to relocate the company can offer you statutory redundancy.
In practice relocations within the country are subject to negotiation between the employees (or their Trade Unions) and the management which may lead to more favourable terms for relocation / redundancy.
I think where the relocation is deemed reasonable (e.g. within Dublin) no redundancy is payable. Some companies however pay money to staff toward the impact of relocating