The governing statute is the Succession Act 1965. Under section 77, you must be 18 or over, or be or have been married, and be of sound disposing mind. Section 78 sets the formalities: the will must be in writing, signed at the foot or end by you, and your signature must be made or acknowledged in the presence of two witnesses present at the same time, each of whom then signs in your presence. Anything written after your signature has no effect. You can read the consolidated text on the Law Reform Commission's revised Succession Act 1965.
Three other rules shape every Irish will. Section 82 makes a gift to a witness, or to the witness's spouse or civil partner, void, although the rest of the will stands. Section 85 provides that a later marriage or civil partnership revokes the will unless it was made in contemplation of it, while divorce does not. Finally, sections 111 and 111A give a surviving spouse or civil partner a legal right share of one-half of the estate if there are no children, or one-third if there are children, which has priority over the gifts in the will.
Children have no fixed share, but under section 117 a child of any age can ask the court for provision if the parent failed in the moral duty to provide for that child. The application must be brought within six months of the first grant of representation. Guardianship is governed separately by section 7 of the Guardianship of Infants Act 1964, under which a parent who is a guardian may appoint a testamentary guardian to act jointly with any surviving guardian of the child, with the court deciding if the two disagree.