i grow outdoor in ireland every summer.. all you need to do for a sucessful grow is buy seeds that will grow in a colder enviroment (ask your headshop for help with this), i would recomend kc33xmasterkush from nirvana seeds, big bang feminized from greenhouse seeds co., or early girl... i've had sucess with all these strains for the last 3 years and would recommend for anyone interested in growing outdoors in ireland.
another good thing to do is make sure your plants get a good few hours sunlight a day.. if your back garden does'nt get good sunlight just look around your local area and parks for good, remote grow spots.. where i grow outdoors is a forest near enough to my house.. to get a good amount of sunlight you will probably have to break aload of trees down.. what i done was put my plants on the top branches of trees in hanging baskets..
all you need to worry about after picking a suitable strain and a good plot is decent soil, perlite and some good feed and grow..(i would recommend iguana juice which is available in hydrophonics).. the more time and work you dedicate to your outdoor grow the more you will be repayed with lovely potent buds..
happy growing