In the last Census, in 1996, more than 30,000 couples described themselves as living in a cohabiting family unit. Of those about 40 per cent had children. This figure is likely to have increased significantly since then, when the question was asked for the first time.

However, the legal rights of such families are virtually non-existent, and the partners in such relationships face problems of nightmare proportions if the relationship breaks down, or when one partner dies. The only way to avert them is to ring-fence various aspects of the relationships with legal agreements - or, of course, to get married.

"People think there is such a thing as common-law marriage, and there isn't," says Muriel Walls, solicitor with McCann FitzGerald. "The term arose because, prior to 1753, there were no minimum legal requirements for marriage, and there were instances of secret marriages contracted without parental consent. So a law was introduced spelling out the conditions for a legal marriage, including that it take place in public, before witnesses, with banns etc. That effectively abolished common-law marriage."

Therefore, even if a couple have been living together for many decades, the law treats them as "strangers" for the purpose of taxation, property, the family home, maintenance and inheritance. These issues arise in two circumstances: if the relationship breaks down, and when one partner dies.

In the event of the relationship breaking down, there can be a problem with the family home. A family home is defined in law as "a property in which a married couple ordinarily resides". Therefore, the home of a cohabiting couple is not covered, and, unless it is in their joint names, the partner whose name is not registered has no claim whatsoever on the property if the relationship breaks down.

"There should be a co-ownership agreement if the couple is buying a house together," says Walls. "For example, you could have a situation where the parents of the girl would put in, say, £10,000, and then things go wrong. There might be no record of that contribution."

The individuals may then attempt to sort out the ownership of the property in the courts. Without receipts registering every single transaction in the household, it can be difficult to sort out what claim each party has to the property.

It may also be worth bearing in mind that these issues are argued out in public. The protection of the in camera rule for family law cases does not apply to disputes between non-marital couples, except where children are involved - the President of the Circuit Court recently ruled that such disputes be dealt with in the family courts. Otherwise they are dealt with under the 140-year-old Partition Acts, where property is divided according to what each party put in.

Further problems can arise for cohabiting couples with children. Nonmarital fathers have no automatic parental rights. The legal situation of fathers living with and caring for their non-marital children is exactly the same of that of "disconnected" fathers, who play no role in their children's lives.

This can easily be rectified by the mother nominating the father as the guardian of the child or children. "Those rights are there for the asking," says Walls. "Those in happy relationships are often unaware of the situation. But if the relationship breaks down, or if the mother becomes ill, then problems arise."

David Bergin, another solicitor specialising in family law, points out that the law recognises that a spouse makes a contribution through work in the home and raising children, but there is no such recognition for an unmarried partner. Nor does a dependent partner in a cohabiting relationship have maintenance rights.

For example, if a couple has been living together for 20 years, with the woman working in the home and caring for the children of the relationship, and the relationship breaks down, she could find herself destitute. She will have no automatic rights to a share in the home, and she will not be entitled to maintenance. Children are entitled to maintenance irrespective of the marital status of their parents.

The situation can be even worse when one partner dies. If the family home has not been registered in both names, the surviving partner will not inherit it. Even if it has been registered but there is no will, he or she has no right to the other partner's share, and could find it claimed by a sibling or other relative.

BUT even a will does not solve all the problems. Because the partner is legally a "stranger", he or she will be liable for inheritance tax. The threshold before this tax kicks in is £12,000.

In last year's Budget, Finance Minister Charlie McCreevy did introduce some measures to protect the inheritance rights to the home of cohabiting couples. If they have lived together for five years prior to the death, and the surviving partner continues to live in the family home for six years, tax is not levied on the inherited share. This provision also covers siblings or same-sex couples living together.

But there is no such protection for the inheritance of other property. A spouse is automatically entitled to inherit a third of the other spouse's property, and can get more in a will. He or she is exempt from taxation. But a cohabiting partner has no such rights, and if she inherits property, such as a business, in a will, she will have to pay heavy inheritance tax.

Couples may try to avoid this by transferring property while alive. Caught again! Mr Bergin points out that this entails stamp duty and gift tax or capital gains tax. Indeed, even placing the home of one partner in the couple's joint names will entail stamp duty and gift tax.

Asked what advice he would give to a cohabiting couple, Bergin laughs and says: "Get married! Otherwise pay up".

There is just one area where the law does provide some protection for cohabiting couples - where there is domestic violence. Protection, safety and barring orders can now be sought against a violent partner, and also against a violent adult son or daughter. The number of applications for such orders against an unmarried partner has been rising steadily since the legislation was introduced.