Hi Mark,
I hope you're well.
Basically, the canonical tag is used to let Google know which URL it should refer to as the original source of the page content. So, if you had the following URLs that all go to the homepage:
www.domain.com/
www.domain.com/index.php
www.domain.com/home/
Then Google could crawl each of these pages and identify them as three different pages all with the same content. This could say to them that there is duplicate content on the site (which is not good). Usually with the homepage Google is intelligent enough to understand that there is just one page and the /index.php for example isn't a duplicate.
The problem that you do face, especially on the site that you are optimising, is with the different pages that have information on the lettings, etc (i.e. your product pages). For example, if you look at the following URL on your website:
http://www.daft.ie/searchshortterm.daft?id=23606
This is when you go through to the short-term searches and then I find the '176 Rathgar Road' apartment. Due to the dynamically generated URL (search.shortterm.daft?id=23606) I can gather that there would be several ways to get to this page with a different URL. My first suggestion would be to set up Search Engine Friendly URLs, for example, instead of having 'http://www.daft.ie/searchshortterm.daft?id=23606', it would be:
http://www.daft.ie/short-term/dublin/176-rathgar-road-apartment/
This way you could clearly optimise the page on Google search and have the canonical link to the page as:
href="http://www.daft.ie/short-term/dublin/176-rathgar-road-apartment.html" rel="canonical" />
This would improve the SEO performance on the website and avoid duplicate content issues.
I hope this helps, but if you need any more info then just let me know.
Matt.