A few questions re. internal doors (I've checked the key posts already)
* I'm in Cork, has anyone used Costello Doors?
There is a company in Limerick that have a fantastic range Mid West doors, you can also pick out your glass there and they will fit this also, they have an excellent range of door and glass designs and also sell prefinished. They are located in Mulgrave St. across the road from the Fire Station and as for as I know they are doing a 30% off sale at the moment 
* How difficult is it to varnish doors? Better to hire a painter?
Not that hard all that's need is patience, clean the door with white spirits to remove any dirt and pencil marks then lightly sand, dust off and treat with first coat let it dry completely, then lightly sand again and apply second coat if you like you can apply three coats but in my own home I found two to be suffiecient. You can also buy prevarnished doors although these are more expensive.
* What's the difference between a pine door and a mahoganey door after
they've being painted? They both look similar to me when 'raw'.
Mahogoney is a dark hard wood and will stand up to much more punishment, pine/red deal is a light coloured soft wood but looks great when treated with clear satin finish varnish.
* Is leaded glass really needed in a door?
No you can have pretty much any type of glass you like.
Thanks