Choosing trees and shrubs for your garden is a very personal matter, so the first thing you ought to do is consider your own preferences.
Make a short list of the types of plants you're interested in and this will form your starting point. Once this is completed, you're then ready to do some research to make sure your choices are going to be compatible with your environment.
1. Choose trees or shrubs that are suited to your local climate
If you live on an Irish mountain, the chances of successfully raising a desert cactus farm in the great outdoors are not exactly going to be encouraging. Nor will your date palms and coconut trees be likely to flourish.
Just be realistic. Trees that grow on similar latitude to your own are a good start, but also consider the rainfall and soil type. If you have a mainly loamy soil, plants that grow better in sandy soil won't cope as well, and may fail.
Good choices for Ireland include garden trees that grow well in northern Europe, the UK, North America, China, Nepal, Bhutan, Mongolia, Korea, and Japan. Trees from places like Australia or Africa are less likely to be successful, and that's even if you can obtain them.
Those trees and shrubs that are native to the South Island of New Zealand and to Tasmania are also potential candidates, as they can survive the high rainfall and cold temperatures of an Irish winter. However, as evergreen trees, they won't appreciate snow as these trees don't have the ability to become dormant.
2. Use some tricks to protect delicate species
If you have a tree or shrub that you like but it's not especially suited to your home climate, you can try protect it from the elements by using garden hedges or even buildings to provide a bit of shelter. A hedge can provide some insulation; protecting against wind, frost, and other climate types.
Likewise, tall trees can provide some shelter for shrubs planted below their canopy.
3. Choose a theme for your garden
There are many different themes for gardens, although some may be less suited to your environment than others.
For example, a Japanese garden (not one of those zen arrangements that's just stones, however) can be successful in a northern climate. These will include trees like cherry trees, cedar, Japanese black pine, Himalayan white pine, willow, maple, and water oak.
To make a hedge for a Japanese garden, the most traditional choice is Japanese barberry, followed by Japanese pittosporum. A more readily available plant in Ireland is yew, but you may need to be careful if you have livestock as yew trees contain toxins that could affect animals which graze on them.
Before you rush out to start creating a Japanese garden, however, think about how much sense it's going to make in your location. If you've only got a tiny amount of land, it could seem slightly eccentric to plant a Japanese garden.
On the other hand, if you have a huge sprawling yard, devoting one section of it to such a garden could be a wonderful idea.
4. Think about how much time your have to devote
Often the most beautiful plants are those that need the most maintenance and care. If you're a busy person, you'll want to choose hardier species that can stay healthy with less regular care and attention.
Just remember that a garden is meant to be enjoyed. The most important thing is to make sure you end up with the garden you want, featuring plants you like, and where attending to the needs of the garden will always be a labour of love, never a chore. You know you have made the right choice when you find pleasure in gardening.