Hi Gareth. The best advice I can give you is to make sure you are using the 20mm eyepiece on its own. do not use the barlow just yet. This should give you around 43-44 times magnification, and will give you the best chance to find objects. Also, try to line up the finder scope with the main scope as closely as you can. Do this in daylight if possible. Point the main telescope at a distant object, like a chimney, top of a tree or lamp post, something like that. Then adjust the small screws on the finderscope so that you can now see the same object in both the main telescope and the finder.
Hopefully you will now be able to use the finderscope to find the Moon, and will also be able to see it in the main scope at the same time. Adjust focus as required until you get as sharp an image as possible. You can now add the barlow to the 20mm eyepiece, putting the barlow into the focuser first, then the eyepiece. This should give you around 86-88 times magnification.
I'm sorry to have to tell you that it is unlikely you will be able to use the 4mm eyepiece. This, on its own, would give 220 approx times magnification in your telescope, and around 440 times magnification if used with the barlow. The fact is that, in Irish skies, it is very rare that magnification over 120-150 times can be used, even in the very best of telescopes. This is because of constant disturbances in the atmosphere. Lastly, I would encourage you to join a local astronomy society if there is one near you. Take advice from people there, I'm sure someone will be glad to give you further advice.
Good luck
