Within Writer: To check a word in the text, first select it (with the mouse), and then:
Or, you can use the shortcut: CNTL + F7
If the steps have been followed correctly, a window such as the one below will appear (This particular one is taken from Open Office, which in this case is the same.)
Note: You can also open the thesaurus without first selecting a word from the text.
Note: There is no thesaurus function (for Irish) in Microsoft Word.