Since Ireland and the United Kingdom do not take part in all the provisions of the Schengen acquis, they are in a special situation, which the Protocol (No 19) on the Schengen acquis integrated into the framework of the European Union (the Schengen Protocol) took into account in two respects. First, Article 4 of the protocol reserves to those two Member States the right to apply at any time to take part in all or some of the provisions of the Schengen acquis in force on the date of the application to take part. Secondly, Article 5 of the protocol, which governs the adoption of proposals and initiatives to build upon the Schengen acquis, allows those Member States to choose whether or not to take part in the adoption of such a measure, with that option being available to one of those Member States only if the context of the measure is an area of the Schengen acquis which that Member State has accepted pursuant to Article 4 of the protocol, or if the measure is a development of such an area.

In that context, while, pursuant to Article 4 of the Schengen Protocol and to Decisions 2000/365 concerning the request of the United Kingdom of Great Britain and Northern Ireland to take part in some of the provisions of the Schengen acquis and 2002/192 concerning Ireland’s request to take part in some of the provisions of the Schengen acquis, Ireland and the United Kingdom take part in certain provisions of the Schengen acquis, that participation does not extend to the provisions of the acquis relating to the crossing of the external borders. Ireland and the United Kingdom can therefore take part in the provisions in force of the Schengen acquis relating to that area, or in the adoption of proposals and initiatives to build upon that acquis which relate to that area, only after a request to that effect has been made by the Member State concerned and accepted by the Council deciding in accordance with the procedure laid down in Article 4 of the Schengen Protocol. It follows that the EU legislature cannot validly establish a procedure that differs from that provided for in Article 4 of the Schengen Protocol, whether in the direction of strengthening or easing that procedure, for the purpose of authorising Ireland or the United Kingdom to take part in such provisions or in the adoption of such proposals and initiatives. Similarly, the EU legislature cannot give Member States the possibility of concluding agreements between themselves having such an effect.

As regards Article 19 of Regulation No 1052/2013 establishing the European Border Surveillance System (Eurosur), which makes provision for establishing cooperation for the exchange of information relating to the crossing of the external borders on the basis of bilateral or multilateral agreements between Ireland or the United Kingdom and one or several neighbouring Member States, that provision cannot be regarded as giving the Member States the option of concluding agreements which allow Ireland or the United Kingdom to take part in the provisions in force of the Schengen acquis in the area of the crossing of the external borders. The agreements mentioned in that provision allow the implementation of a limited form of cooperation between Ireland and the United Kingdom and one or several neighbouring Member States, but cannot place Ireland or the United Kingdom in a situation equivalent to that of the other Member States, in that those agreements cannot validly lay down rights or obligations for those two Member States comparable to those of the other Member States in connection with the Eurosur system or a large part of it.

It follows from the general scheme of the Schengen Protocol, from Declaration No 45 on Article 4 of the Protocol integrating the Schengen acquis into the framework of the European Union, and from the principle of sincere cooperation that the system established in Articles 4 and 5 of the Schengen Protocol cannot be regarded as intended to require Ireland and the United Kingdom to participate in the entire Schengen acquis, excluding any form of limited cooperation with those Member States. Moreover, to interpret Article 4 of the Schengen Protocol as not applying to limited forms of cooperation does not call into question the effectiveness of that article, in so far as that interpretation does not allow Ireland and the United Kingdom to obtain rights comparable to those of the other Member States as regards the provisions in force of the Schengen acquis, or to take part in the adoption of proposals and initiatives to build upon the Schengen acquis, without first having been authorised to take part in those provisions by a unanimous decision of the Council on the basis of that article.

Similarly, the fact that the Member States participating in the Schengen acquis are not obliged, when they develop and deepen the enhanced cooperation which they have been authorised to establish by Article 1 of the Schengen Protocol, to provide for special adaptation measures for the other Member States does not mean that the EU legislature is prohibited from enacting such measures, in particular allowing certain limited forms of cooperation with those other Member States, where it finds it appropriate. Furthermore, the fact that the establishment of limited forms of cooperation may lead to fragmentation of the rules applicable in that area, assuming that to be the case, cannot call that conclusion into question, as the implementation of enhanced cooperation inevitably leads to a certain fragmentation of the rules applicable to the Member States in the area concerned. It follows from all the above considerations that limited forms of cooperation do not constitute a form of taking part within the meaning of Article 4 of the Schengen Protocol.

(see paras 27-33, 42, 51-53, 55, 58, 59)