You should really go D-U-D-U all the time. This is called "alternate" picking. If you do a down stroke then you do a hammer on or a pull of then your next pick stroke should be a down stroke as you need to account for the upstroke where you decide to do a hammer on or pull off verses picking that note.

1/4 notes are down strokes, 1/8 notes are an upstroke.

Alternate picking is the best way to learn if you want to get good at an instrument and play up to speeds of say 240 bpm. Once you have the discipline of alternate picking then you can go to more of an "economy" or "efficient" style picking where you say do a down stroke on one string then go up a string (which is the direction down) then you could simply do another down....so it would be D-D when moving to a string lower than the one you just pick. This is frowned upon by the "purists" Once you do three down strokes across three strings, or three up strokes across three different strings, this is really referred to as "sweeping".

Try it with just open strings so that you get used to pattern, without having to worry about the specific notes of the jig. It also might help focusing on rhythm playing, rather than melody lead, where it might seem more natural to strum DUD DUD. You also might try a classical mando method, like Marilynn Mair's, that emphasizes controlling pick direction (beyond DUDU and DUDDUD).