Just took a look at your site and have to note that I liked the style and design.
As for the online album: why not try setting up a sub-domain like http://clients.johnnycorcoran.ie” onclick=”window.open(this.href);return false; , where you can actually make a fresh installation of e.g. Gallery 2 or use the solution by robert. Another option (haven’t explored it in detail) is using Picasa Albums, in which you would define access to certain e-mail addresses; however, there would be several drawbacks, namely, lengthy address and clients would have to deal with Google registration, etc.
All in all, I would suggest putting just a link on your current page that leads to clients area or use a sub-domain that is easy to remember.
I think Gallery2 could do some of the work. I usually tried to set it up along with some blog, like Wordress, but it should work standalone fine as well. Another option is to add some kind of extension (e.g. e-commerce plugin) to WordPress or Joomla site.