A dissertation is a long piece of written work (think of your assignments, but much longer) composed of original research and putting forward a specific argument or point of view. Dissertations are normally written by students are taking degrees, especially postgraduate degrees. A thesis is very similar to a dissertation; some might they are identical and the terms are often used interchangeably. If there is a difference in Ireland, then it would be that a dissertation would be written as a part of an undergraduate or Master's degree, while a thesis would be written as part of a PhD.
Dissertations and theses are often freely available online and are useful both as pieces of original work in themselves and as sources of useful information about a topic as they will contain reference lists and bibliographies. The Library's resource, Education Database contains many theses and dissertations of the topic of education
