Are you married? If so, you already have Joint Custody. You cannot lose custody unless the court makes a declaration that you are "unfit to have custody of any dependent child of the family
". Section 41.-(1)
JUDICIAL SEPARATION AND FAMILY LAW REFORM ACT, 1989
Just be careful as the Court will seek to water this down by granting you and your wife Joint Custody
"with main care and residence to the mother
As a married father, you are also an automatic guardian which means that all decisions regarding the children should be made JOINTLY as per Section 6.-(1)
of the Guardianship of Infants Act 1964.
According to S.I. No. 5/1998
GUARDIANSHIP OF CHILDREN (STATUTORY DECLARATION) REGULATIONS, 1998, Guardianship is "The collection of rights and duties which a parent has in respect of his or her child. It encompasses the duty to maintain and properly care for the child and the right to make decisions about a child's religious and secular education, health requirements and other matters affecting the welfare of the child. The exercise of guardianship rights may be agreed between parents. In the event of a dispute arising concerning the exercise of guardianship rights the court may determine the matter on the application of either parental guardian
If you are not married, it's a whole different ball game.
I suggest that you write to your ex and suggest mediation before making an application to Court.