Photo of a pot/grow bag dripper kit being used in a polytunnel
The best time to water your polytunnel, as with elsewhere in your garden, is in the evening. Plants with deeps roots are able to pull water up from deeper layers of the soil, but for smaller plants or those with shallow root systems (such as lettuce) watering in the evening allows more time for the plant to draw up more water than a morning application. 
There are many ways in which this can be done, the humble watering can, of course, is an invaluable asset. But no matter how sophisticated your watering system is, some plants may require a little more water than others, and this can be very labour intensive during the summer months. This is why most gardeners choose a combination of watering methods to help them fine-tune the moisture for each plant.
Here is a brief breakdown of some popular polytunnel irrigation methods:

Overhead Sprinklers

These are common in commercial tunnels, mostly because they are easy to install and run provided you have mains access. Overhead sprinklers can be installed with a timer, allowing you to control when and how long they will water your tunnel for, this is ideal if you are going on holiday or do not have time to water your polytunnel every day during the height of summer. The downside to these sprinklers is that they water indiscriminately to all plants, this is fine if you only plan on growing one kind of crop, but not if you are planning to grow a variety of plants.

Porous Pipe/Drip Line

Drip lines are a common solution to the time-consuming nature of watering a polytunnel. Like overhead sprinklers, they water indiscriminately, but if they are laid out just below the surface of the soil or under a generous layer of mulch the water takes longer to evaporate. Drip lines are lengths of porous rubber pipe, designed to allow water to seep through slowly. They are an ideal low-level watering system, placing the water where it is required, without wetting foliage and flowers or soaking the pathway. 

Pot/Grow Bag Dripper

Pot and grow bag dripper kits are watering systems that place the water precisely where it is required. Each dripper can be stabbed into the soil of a pot or grow bag and the steady drip of water will keep the soil moist without wetting foliage and flowers. This protects against scorching and is economical for water consumption. The drip lines can be turned off and on manually via a mains supply attached to a timer.