Was it just one man? Or numerous men? I'm also curious as to how they got bombs into mainland England. And one last thing why were the South Armagh brigade so hard to deal with for the British army, I heard a lot about them and how British army forces wouldn't even enter certain villages.
Early on the bombs were mostly homemade using Ammonium Nitrate fertiliser. While the process is simple it does take skill to get it right and there were several people who specialised in it. The explosives were often made in ordinary houses or sheds.
The provision of Semtex from Libya in the 1980s gave the IRA a lot of flexibility as it is more powerful and useful.
Richard English, Armed Struggle is probably the best history of the IRA. You could also look at James Bowyer Bell or Tim Pat Coogan. Coogan is a journalist with good sources but his historical analysis is very poor so be warned.