There are several circumstances in which you can use copyright protected works without having to obtain the permission of the copyright owner. The Copyright and Related Rights Act 2000 contained a number of copyright exceptions, which have been modernised and expanded in the 2019 Act. This also introduced new, education-related exceptions reflecting the development and expansion of online learning and the use of digital content.
In order to infringe the copyright, a “substantial” part of the work must have been copied. However, how do you define substantial? How does the copyright holder or the lawyers define substantial? Is the copying of one page of a 500-page book a "substantial" breach of copyright? It probably isn't. What about 10 pages? 50? 100? Copying of small amounts of a copyrighted work is permissible but it's impossible to provide a definition that covers all situations, but lawyers might like to argue about it in court.
Fair dealing is a concept in common law jurisdictions such as Ireland and the UK and refers to situations where you don't need to ask permission from the copyright holder to use their work. In America, the idea is called "fair use". Fair dealing is an exception to the exclusive rights of copyright owners to control use of their work and is not a right in itself. Instead it is more of a defence against a charge of copyright infringement. Fair dealing applies when you provide satisfactory attribution or acknowledgement to the copyright holder and:
Fair dealing applies on a case-by-case basis, and the following will be considered:
In additional there are exceptions for educational use (discussed on the Copyright and Education page) and for libraries and archives, which have limited rights to use copyrighted works under certain conditions
Note: making multiple copies of something (e.g., to distribute in a class) does not qualify as fair dealing.