Dublin's layout
The first thing to understand about Dublin is that it is divided into north and south sides by the River Liffey. Postal codes -- if you want to call them that -- consist of single and double digit numbers. Even numbers are on the south and odds on the north. (With the rivalry between north and south siders, no doubt a south sider came up with that number placement.) Hence, if you are visiting a friend in Dublin 7, you know they live on the north side. The further away from city centre the neighbourhood is, the higher the number, so Dublin 13 is way to heck and gone if you're taking the bus from O'Connell Street.

Don't trust steet names -- they change almost every block. Ask for landmarks when getting directions and while you're at it, try to get a sense of how far something is. When the Irish say, "you follow that street for awhile" you could be in for a very long walk.