Sometimes I ride in the park which has no light. There always are dog walkers, runners and other cyclists. So, when I ride towards someone I have two options:
My front light is: 
So, the question is: how to be less irritating?
Consider how you feel when a fellow cyclist approaches you will full beam straight ahead. It's pretty blinding, even for a moment, and especially off-road when your eyes aren't used to it.
So in a park:
If you do this then it doesn't matter whether you illuminate pedestrians or for how long.
There are now many bike lights on the market which have a shaped beam with a "horizontal cutoff" giving strong light onto the road or path, but much less above the horizon. When adjusted correctly these allow you to see where you're going without dazzling oncoming traffic or pedestrians.
I use a Busch & Muller Ixon IQ (pictured), but there are others.
The gold standard for road lighting that doesn't dazzle oncoming road users is StVZO. It's a German standard, but IMO sensible to use on roads and busy paths anywhere.
If you don't want to fork out for a new light, you can make a hood for the light to block the light from going upwards.
I had to face exactly this on an old commute. Dipping the front light was absolutely necessary in a park and another stretch of unlit bike path. It was a bright enough light to illuminate the road, though not well enough to ride at any decent rate on low power.
The solution I found to this was to add a narrow-beam head torch. This can be dipped hands free, and points where you look rather than where the bars are pointing (quite useful when it's pitch dark and you're next to a lake going round sharp bends). Overhanging branches can mean the need to illuminate stuff that's not the road surface, so I tended to adjust the light as I rode much as you'd switch from main beams to dipped beams in a car.
The head torch I bought was ~5 from dealextreme, and is 150 lumens nominal -- so not very much ligfht in total. But all that light is in a bike-sized area of road even if it's directed 10 bike lengths away. I tended to adjust the angle such that I had to tip my head slightly up to illuminate as far away as I'd like for ~30km/h speeds on imperfect tarmac, so that a natural head position was more suited to slower speeds with other people around. In busy areas well-lit I could easily point it almost straight down.
In case you can read German, here's a nice explanation how to adjust the bike front light.
Summary:
make sure that the upper rim of the light at 5m distance (wall) is below that height.
Official German regulation: center of light beam should be at half height at 5 m distance = should hit the road at 10 m distance
Adjust speed according to visible range (just like when driving a car)
You can dip the light slightly (point it towards the ground). Even without considering politeness, you might prefer to illuminate the ground ahead of you, instead of (as you would on a lit city street) pointing it straight forward to be seen as maximum distance.
Lights or not, when you're mixing with people walking, just go slow. I'd be much more annoyed to be overtaken at speed than to have a light shined on me, because your speed is an actual danger, and the light, at worst, an irritant.
I wear an adjustable head strap flashlight at night while riding my bike. The one I have has three colors (white, red and green)which allows me to use a less intense color at times, so as not to blind people. As an addition to three colors, of has three level of intensity. It is worth looking into. With this you are able to turn your head slightly when needed.
These examples may be of help to you. The one pictured is the style I use at the moment.
how to be less irritating?
Some countries have legal requirements around what makes for a good bike light, for example Germany.
Cheap lights are almost always:
These cheap lights are excellent in blinding oncoming people. The only possibility you can see forward is to direct the brightest part of the beam far away in the road. This means a large portion of the spherically symmetric pattern is projected above the horizon, blinding oncoming riders.
However, the cheap lights are usually poor for seeing because the beam pattern is not designed for road use. The spherical pattern directs too much light where not needed and too little light where needed.
The Germans have a solution for this: their StVZO laws require lights to be non-blinking, powered by a power source that doesn't suddenly go away (=dynamo), and the beam pattern to be such that it lights the road ahead of the cyclist and only the road ahead of the cyclist, having a sharp cut-off at the beam to not direct any part of the bright beam above the horizon (there will be still plenty of spill light that won't blind others but allows car drivers to see there's a bike).
Unfortunately quality costs. Only about 1% of people live in Germany, so pretty much the Germans are designing lights only for 1% of the population. If a German quality headlight manufacturer can amortize the product development costs of a true vehicle headlight over 1% of cyclists in this planet and a Chinese manufacturer can amortize the product "development" (in quotes because reusing flashlight technology doesn't really require any additional work) costs of a flashlight over 99% of cyclists in this planet, guess which light will be more expensive and which will be less expensive?
There are lights for illuminated streets and lights for rough terrain in the darkness. They best features are not the same. One case calls for low, narrow and not too bright beams, and in another such approach may just not illuminate your path if the turn is sharp enough or terrain is too rough.
This is probably the most that can be done. The highly shaped beam is actually dangerous in a wrong place, so probably not good to say you absolutely must have such a light, unless maybe it provides a separate "high beam" option. In any case, be sure first you always see far enough to stop, then think about politeness.
