1. All persons may be joined in one action as plaintiffs in whom any right to relief in respect of or arising out of the same transactions is alleged to exist, whether jointly, severally, or in the alternative, where, if such persons brought separate actions, any common question of law or fact would arise; provided that if, upon a motion on notice by any defendant, it shall appear that such joinder may embarrass or delay the trial, the Judge may order separate trials, or make such other order as may be right, and provided also that no person shall be made a plaintiff without his consent.
2. All persons may be joined as defendants against whom the right to any relief is alleged to exist, whether jointly, severally, or in the alternative. When the plaintiff is in doubt as to the person from whom he is entitled to redress, he may join two or more defendants, to the intent that the question as to which, if any, of the defendants is liable, and to what extent, may be determined as between all parties. It shall not be necessary that every defendant shall be interested as to all the relief sought, or as to every cause of action included in any proceedings against him; but the Judge may make such order as may appear just, to prevent any defendant from being embarrassed, or put to expense, by being required to attend any proceedings in which he has no interest.
3. The plaintiff may, at his option, join as defendants to the same action all or any of the persons severally, or jointly and severally, liable on any one contract, including parties to bills of exchange and promissory notes.
4.  No action, cause or matter shall be defeated by reason of the misjoinder or non-joinder of parties, and the Judge may in every action, cause or matter deal with the subject in controversy so far as regards the rights and interests of the parties actually before him. The Judge may, at any stage of the proceedings, either upon or without the application of any party, and on such terms as may appear to him to be just, order that the name of any party, whether plaintiff or defendant, who has been improperly joined, be struck out, and that the name of any person who ought to have been joined as a party, or whose presence before the Court may be necessary in order to enable the Judge to adjudicate upon and settle all the questions involved in the cause or matter, be added as a plaintiff or a defendant. No person shall be added as a plaintiff suing without a next friend, or as the next friend of a plaintiff under any disability, without his consent in writing thereto. Every person whose name is so added as defendant shall be served with a Civil Bill in such manner as may be prescribed by any Order of the Court, and the action, cause or matter, as against such party, shall be deemed to have begun only on the making of the Order adding such party.
5. An infant may sue by his next friend, and in such case there shall be lodged with the Civil Bill a written authority authorising the commencement and prosecution of the suit on behalf of the infant and the use of the name of such person as next friend, signed by such person.
6. An infant may defend by his guardian ad litem appointed for that purpose in accordance with Order 19 of these Rules.
7. When proceedings have been brought on behalf of or against an infant, the Court may appoint a next friend or guardian ad litem to act for or on behalf of such infant, and may change any next friend or guardian ad litem, and appoint another in his place, and may also direct that any money or other personal property to which such infant may be declared entitled in such proceedings be secured or invested for the benefit of such infant in such manner as the Judge shall consider advisable.
8.  (1) In this rule, the “2015 Act” means the Assisted Decision-Making (Capacity) Act 2015, and expressions used have the meanings assigned to them by the 2015 Act.
(2) Where there is in force in respect of a relevant person a decision-making representation order which authorises a decision-making representative appointed in respect of the person to institute, prosecute or defend such proceedings, the relevant person shall sue or defend by his or her decision-making representative, and service on the decision-making representative shall be good and sufficient service.
(3) Where there is in force in respect of a relevant person as the donor a registered enduring power of attorney in respect of which a notification of lack of capacity has been accepted, which includes authority to act on the donor’s behalf in relation to the institution, prosecution or defence of proceedings, the relevant person shall sue or defend by his or her attorney, and service on the attorney shall be good and sufficient service.
(4) Where there is in force in respect of a relevant person as the donor a registered enduring power under the Act of 1996, which includes authority to act on the donor’s behalf in relation to the institution, prosecution or defence of proceedings, the relevant person shall sue or defend by his or her attorney under the Act of 1996, and service on the attorney under the Act of 1996 shall be good and sufficient service.
(5) Where there is in force in respect of an intending party or party to proceedings as the appointer a registered co-decision-making agreement which provides for the making jointly by the appointer and a co-decision-maker of decisions concerning proceedings, the originating document or defence, and each subsequent document delivered in the proceedings shall be signed by, or shall include a certificate that the decision to issue and deliver same was made jointly by, the appointer and co-decision-maker.
(6) Where any issue arises as to, or arising from, the capacity of any adult party to proceedings, the Court may, of its own motion having heard the parties, or on the application by motion on notice of any party, or person sufficiently interested, make such orders or give such directions concerning the further conduct of the proceedings as are necessary in the interests of justice.
(7) A ward of court may sue or defend by his or her committee
9. Trustees, executors and administrators may sue and be sued on behalf of, or as representing the property or estate of which they are trustees or representatives, without joining any of the persons beneficially interested in the trust or estate, and shall be considered as representing such persons; but the Judge may, at any stage of the proceedings, order any of such persons to be made parties either in addition to or in lieu of the previously existing parties. This rule shall apply to trustees, executors, and administrators sued in proceedings to enforce a security by sale or otherwise.
10. Save in actions founded on tort, when there are numerous persons having the same interest in one action or matter, one or more of such persons may sue or be sued, or may be authorised by the Judge to defend, in such action or matter, on behalf of or for the benefit of all persons so interested.
11. Where proceedings are taken for the recovery of land, any tenant, under-tenant, or other person in actual possession of the property sought to be recovered, or any part thereof, may be named as defendant and the Civil Bill shall be directed to such tenant, under-tenant, or other person, with the addition of the words ‘‘and all persons concerned’’.
12. Where proceedings are taken as mentioned in the last preceding rule, any person not named in or served with the Civil Bill may by leave of the Judge appear and defend at the hearing, and the Judge may thereafter deal with such person, both as to costs and otherwise, as if he had originally been served with the Civil Bill as defendant.
13. When any person residing outside the Circuit, but inside the State, is a necessary and proper party to a proceeding properly instituted in the Court against some person within the Circuit, such first mentioned person may be served with the Civil Bill or other document without special leave.
14. Subject to the requirements of Rule 9 of Section 66 of the Chancery (Ireland) Act, 1867, wherever, in any action for the administration of the estate of a deceased person, or for the execution of the trusts of any deed or instrument, or for the partition or sale of any hereditaments, a judgment or order has been pronounced affecting the rights or interests of persons not parties to the action, the Court may direct that any persons interested in the estate, or under the trusts, or in the hereditaments, shall be served with notice of the judgment or order; and after such notice such persons shall be bound by the proceedings, in the same manner as if they had originally been made parties, and shall be at liberty to attend the proceedings under the judgment or order. Any person so served may within one month after such service apply to the Court to discharge, vary or add to the said judgment or order.