This has come up alot in conversation recently. In my case, not married, long term relationship. If I die and leave will with 100% going to her, she will have to to pay tax on the inheritance over 15K as she is Group C (i.e. no relation in the eyes of Irish law). Disaster.
Its also annoying from smaller things e.g. car insurance. We share a car, but since we are not married, insurance companies do not accept both of us as equals. Hence, if i'm the policy holder, she is not getting a no claims bonus, as she is classed as an unrelated named person. If we were married, she keep her no claims bonus despite being a named person only. The solution to this is to sell the car back and over between us every year so we keep both no-claims bonuses intact.
Then there is the issue of buying a house together which I wont even get into.
Being in long term relationship without marriage is a disaster from Irish law perspective (at least if you're a man and a woman, as you dont have the options of civil partnership, which is afforded to other members of society.)