First, check your local ordinances. Second, most likely they are not around your locale because growing conditions are not favorable for these plants. Third, get a good knowledge of the plants you are considering. For instance Loosestrife is an invasive plant but you may be looking at a plant which shares its name. Finally, aside from your own property, I would suggest that you have nature continue to provide you with its own decor ideas.

Does Ireland have any kind of native plant preservation association? Such organizations can usually provide sources of seeds, bulbs, etc., or will sometimes provide them themselves to help restore native flora.

I don't know about the laws in Ireland but in the UK digging up of wild plants is illegal.

You are talking about three plants that are a real nuisance and very invasive in Ireland. I don't know whether what you propose is illegal but it's certainly against the guidelines of the Botanical Society of Britain and Ireland. I would suggest that you contact the BSBI and ask for their advice, their webpages have an address for the Irish Officer.

Good to know Landgirl... last thing any of us need to be doing is encouraging invasive species!

That's why I was suggesting natives, though some natives can be invasive, too--thistles and nettles and brambles, oh my! :)

Best to collect seeds from the plants, rather than dig them up. Loosestrife (Lythrum) germinates very easily.

Not sure I want to propagate them now as people seem ambivalent about them, there are many safer ones which are as colourful I can avail of. Thank you all especially those who drew my attention to the possibility of the illegality of it as had not thought of that angle, don't want to go down the eco terrorism road. Many thanks and will follow up on the BSBI lead when I get time.