Control
Picking each note in the direction that seems best at that moment may sound like a good idea, but you soon get lost. Suddenly you don't know where the pick is in relation to the string, and either you have to stop up and check or you play a wrong note (or even miss the strings completely). When you're playing you simply don't have time to think of the basic techniques. That has to run automatically, and to achieve that you have to follow to very fundamental rules:
1. If you play the same thing twice, play it exactly the same way.
2. If you play similar things, play them as similar as possible.
(Btw, these two rules don't apply to irish banjo picking technique only. They are fundamental rules that apply to all playing techniques on all musical instruments.)
Phrasing
Downstrokes automatically gives a louder note than upstrokes. Classical musicians often spend lots of time and effort trying to eliminate this difference to get the smoothest possible playing. In traditional and folk music we don't want to do that. We want to use the difference musically.
The notes in a bar are never played identically. Notes on the beat are louder than notes off the beat and notes on downbeats (that is the first and third beat in four-time) are louder than those on the upbeats (second and fourth beat). The banjo picking patterns are designed to empasize this.

Fourth notes

If we play a full bar of fourth notes, the picking pattern is easy. Simply start with a downstroke and play every other note up and down:

Picking example 1

(In case you wonder: the sign above a note means a downstroke and the means an upstroke.)

Eight notes

Steady eight notes are just as simple. Again play alternating down/upsrtokes starting with a downstroke:

Picking example 2

Mixed rhythms

Phrases with both eight and fourth notes are a bit more complicated of course, but not much. I tend to think of them as regular eight note phrases with some notes left out. That is the hand moves as if it were playing eight notes, but sometimes it doesn't hit the string.

Does that make sense to you? If not, just forget what I just said and look at these examples instead:

Picking example 3

Picking example 4

Picking example 5

Picking example 6

Picking example 7

Exceptions

Of course there are always exceptions to the rules. We have triplets and music in triple time where we've got three notes on each beat. There are also times when we deliberately break the system to simplify string changes or even to achieve a special musical effect.

You can read all about that in the "picking" chapter, but maybe we should leave that for now. This is supposed to be an introduction to the technique after all. Let's go on to the scales for now.