Welcome!
You'll find wildly varying opinions on this forum about the use of soil in tanks. To some, dirted tanks are utterly stupid, and a few of us don't do anything else. Don't let it scare you. I use Miracle Grow Organic, but I have no idea if that's available in Ireland.
Stay away from compost--it's far too rich. You also don't want anything with added fertilizers or any advertisements of "fertilizes for three months" or anything like that. Anything labeled "organic topsoil" or "organic potting soil" should be OK. Commercially sold soils should have an ingredients list on them. If it contains anything that sounds like it came from a laboratory rather than from the ground, don't buy it.
Another option is to use soil from the ground. Take care not to use anything that might have been sprayed with chemicals, and nothing within 50 meters or so of a busy road or driveway, as car exhaust can carry lots of things that are unfriendly to aquatic life. My Sumatra tank has dirt from my back garden, and it's been going strong for almost a year.
Now, a few things about that "hard to manage" bit that PK mentioned. Apologies if you're already experienced with dirted tanks and know all of this. First of all, unless you are going for a blackwater tank, you will want to cover the soil with some sand. I usually do about one inch of soil capped with one to two inches of sand.
Second, terrestrial soils immersed in water are going to go through all sorts of biological chaos, and it takes a few weeks to sort itself out. (Find a copy of Diane Walstad's "Ecology and the Planted Aquarium" for details, if you're interested in that sort of thing) Among other things, organic soils can leach ammonia...but only for a while. Since most responsible aquariists add ammonia to their new tanks for cycling anyway, I've always seen that as a win-win. As soon as ammonia and nitrites are zero and the plants are growing robustly, I consider my tank cycled and I'm ready to add fish.
Finally, it is commonly said that natural soils will wear out over time, their nutrients will get used up, and the plants will start languishing. My expernience with this varied depending on the soil used. I have had a couple of dirted tanks in which the plants--including nutrient-hogs like amazon swords--were still going strong after four years! In others, I have needed to start using fert tabs after a year or so. I haven't done anything like a scientific investigation to determine why this is so. I suspect it's a combination of many variables, specific soil type being the first. (I can't always find my chosen Miracle Grow Organic, so I've tried a lot of other things through the years)
Hope that helps. Do keep us posted and don't hesitate if you have any questions!
Thomas
