WHO: All the main political parties are in favour: Fine Gael, Labour, Fianna Fáil and Sinn Féin. Other groups supporting it include the Irish Council for Civil Liberties; the Union of Students in Ireland and gay and lesbian groups, including Glen.
It is seen as very much a Labour project within government. That means party leader Joan Burton and Junior Equality Minister Aodhán Ó Ríordáin (below inset) will be to the fore.
Justice Minister Frances Fitzgerald said she believes TDs, senators and ordinary members will campaign for what is Fine Gael policy. Others feel that at least some elected representatives will "sit this one out".
Fianna Fáil justice spokesman Niall Collins insisted the party would vigorously campaign and it was also party policy. But there are already signs of tensions. In 2013 party leader Micheál Martin abandoned efforts to get FF backing for the Protection of Life in Pregnancy law.
Sinn Féin may have similar internal tensions to those in Fine Gael and Fianna Fáil.
CAMPAIGN: Proponents will try to keep it to a straight question of fair play. If you believe that gay and lesbian people should have exactly the same rights as heterosexual people, then you must vote 'Yes'.
THE PROBLEM: The issue of children in gay marriages remains to be clarified. These are children who originated in previous heterosexual relationships, or adopted children, or children conceived with assisted reproduction.
All the political parties say they will support the Children and Family Relations Bill, which is due in the Dáil next month. Justice Minister Frances Fitzgerald said it would be passed before the marriage vote takes place next May.
The Government is stressing that it is completely separate from the referendum vote. But it will be hard to sustain that argument.
WHO: There are few nationally known names who are publicly against the referendum, and some on the 'No' side like to characterise the campaign as "David versus Goliath". They will hope to capitalise on this and the negative public mood towards the established political parties.
So far the most vociferous 'No' campaigners are Independent Senator Rónán Mullen (inset left), the Iona Institute think tank, which promotes the place of religion and family in society, and a group called "Mothers and Fathers Matter". The Catholic Church is opposing the referendum, but it is not clear yet how prominent or active a role the bishops or other senior clerics will take in the campaign.
CAMPAIGN: The promoters of a 'No' vote have accused the Government of "over-simplification" and "dishonesty". They object to the Government styling it as about "marriage equality" - which they see as an attempt to make people think a 'No' vote is a vote against equal rights.
Referendum opponents also insist that the Children and Family Relations Bill is central to the referendum issue. This aims to take account of different family models other than father, mother, and children. By extension they argue that the referendum is in fact really about children and the traditional family model. Senator Rónán Mullen argues that if this referendum is passed the State will never again be allowed take legal measures in favour of a child's right to have a father and mother. By changing the definition of marriage to make it gender neutral, no legal provision can be made for "father and mother parenting".
PROBLEM: Public sympathy is a limited substitute for the resources political parties may deploy. The name recognition of 'Yes' proponents will make it difficult for lesser known or unknown 'No' advocates. The limited arguments around children's rights may be difficult to sustain over four months of campaigning. Rightly, or wrongly, they must battle accusations of homophobia.
* Equality: 'Yes' advocates want to keep it simple. If you believe in equality for gay people - you must give them equal marriage rights.
* Oversimplification: 'No' advocates insist this is "dishonest over-simplification". People can vote 'No' without favouring anti-gay discrimination.
* Children: 'Yes' side says this will be dealt with by separate law, the Children and Family Relations Bill, due in the Dáil next month. Privately, some concede it would be much better had it been dealt with earlier. 'No' advocates insist issues such as adoption, custody, and access to children are central.
* Access to airwaves: Current broadcast rules mean both sides have equal access to radio and television. So, they may cancel each other out. Perhaps the real contest will be a "ground war".
* Allegations of homophobia will affect both sides: 'No' advocates will frequently be accused of it. 'Yes' advocates will often be accused of abusing this allegation.