If you go into Merrion Square on Saturdays and Sundays there is a wide variety of artists' works there. Some artists who exhibited there in previous years have gone on to fame whatever about fortune.
Over the years I've bought loads of stuff online from www.allposters.com. They have a massive selection of posters, everything from photographic landscapes to renaissance masters to vintage advertisements. Some are more expensive than others, but overall I'm delighted with the quality and service. They do also have a framing service but it's massively more expensive to get a framed picture shipped, so I've never used it.
You could get anything you order from them framed in a frame shop, but I've always just bought frames and done it myself, as it's much cheaper. Check out the site, you'll have hours of endless browsing