Well my mother has been an artist for the last 6 or 7 years; not as a commercial one, but more out of enjoyment. She's a member of a local art group who between them organise to hold exhibitions every now and then in the local Arts Center. The fact that theres a group of them makes it easier to hold exhibitions and it would be quiet difficult for anyone of them to produce the necessary volume needed to fill a gallery. So first off its proably a good idea to get involved with an arts group or to even consider strating one. The rest of the members will proably be more interested in just painting than making money - but its a good start. Groups like that usually sell there works for between €70 and €250 each; depending on the piece. As regards marketing, it would be a good idea to start a basic website just to display your work; if someone takes an interest, all they have to do is check out the website. Kevin Sharkey, Graham Knuttell and all the other big irish artists have thir own websites.
I don't know the exact laws regarding selling on the street; but it would be another good option- althought it would reduce massively the amount of time you could actually spend produceing new works - whereas if you get your works in a gallery its passive - they can be sold while your not there. One thing about working on the street - your selling prices proably will have to be at the low end of the scale - people will be making more of an inplus buy as opposed to a considered purchase -"Oh that looks nice, €45, not too bad, yeah i'll buy that it'll go well in the living room".