For vitamin D, you can get it from UV treated mushrooms like these:
Not sure if you can find stuff like this in Ireland though. You'd have to be pretty serious about eating these things regularly to keep your levels up anyway and if you are currently or ever become deficient, no food product is going to easily correct that.
You can also find vitamin d fortified plant milks and other products.
I take the vitashine d3 capsules which are made from lichen. They're high dose and fairly cheap.
For calcium and omega 3s, I'd look at chia seeds. They're a pretty good source of both. Google chia pudding recipes. They look weird but they can be delicious. The one downside- chia seeds aren't cheap.
Savi seeds seem to be the hot new omega 3 trend. Vega's marketing them now. I just received some that I ordered and they taste great. They have an omega 3:6 ratio similar to chia seeds but they look and taste like a nut. They're good for protein too, but they too are stupid expensive.
Sesame and especially poppy seeds are also really great sources of calcium (look them up on nutritiondata.com), but they're full of omega 6s so maybe not your best bet as a reliable calcium source.
There's always tofu made with calcium chloride, almonds, fortified plant milks, and of course greens.
If you have a cheap source of pretty much any kind of green and a hell of a lot of time and ingenuity, you might be interested in leaf curd. It's incredibly rich in protein, calcium, iron, vitamin A and other nutrients.http://eattheweeds.com/forum/index.php?topic=656.0
Leaf concentrate (l/c) is a nutritious food made by mechanically separating indigestible fiber and soluble anti-nutrients from the protein, vitamins, and minerals in fresh green leaves...The nutritional breakdown from the original work was that 20 grams of powder would provide 12 grams of protein. The amounts of vitamins and minerals (such as 440 mgs of calcium and 49 mg of iron) are impressive.
It takes a lot of greens and a fair bit of time to make it though.
You should definitely take a supplement (or eat supplemented food like nutritional yeast, soymilk, etc) for b12. You don't want nerve damage. There are studies showing that there may be enough b12 in a few vegan foods (eg laver, chlorella) to stave off a deficiency at least for a while but the evidence is dicey and it's just not worth any risk. I eat a lot of nutritional yeast and take a megadose supplement every few days.