The following is the list of non Schengen countries in Europe, which had shown interest to join the Schengen area but as of December 2018, they are still not part of Schengen state agreement.
List of Countries Outside Schengen Area
8. Bosnia & Herzegovina
12. Republic of Ireland
13. United Kingdom (UK)
You may be interested to find out which countries you can travel to without visa if you are holding Schengen visa.
You may also like to learn about different types of Schengen visas that are issued to the visa applicants.