Tony i've got good news and bad.
the bad news is that i get the feeling your house was poorly constructed.
not enough insulation was used, walls where not filled in and sealed behind where the floor meets the walls. I could go on..
But its the same with the vast amount of houses in ireland, due to once again very mickey mouse building regulations.
Now the good news!
I am an Interior Designer, but before that I was a professional sound engineer working in a well Known recording studio.
The reason you can hear everything next door is because the sound is vibrating through solid points of the building (someone walks on the floor next door, sound goes down along their floor joist, through badly filled bit of wall, along your floor joist) where its amplified. You need to break and dampen those points.
any or all of these will really help.
Build 2 stud walls with a gap between them, offset to each other, packed with Rockwool (bit like attic wool, made by the same co.)and on a strip of cork tiles(3 high, 4 inch wide, floor and ceiling). cork is very good sound insulator.
Fill in the gaps behind skirting boards etc with soft insulation.
lay a good carpet or wood flooring with very good insulation. you want to disrupt the vibrations as much as poss.
these are just a few low cost ideas. hope it helps. let me know how you get on.
alan - interior designer