Cavies are made to live on grass. Their molars don't stop growing in order to protect them from the microscopic crystals and fibre threads that would otherwise wear them out quickly. Since they cannot deal with cell walls, their intenstines provide ideal home for microbes that can digest cellulose.

The most ideal forage for pet cavies is fresh grass, but unfortunately it's generally impossible to feed them year-round. The second best option is dried grass, called hay.
When fresh grass is available, it's very beneficial to collect them as much as you can. The spieces belongs to the Poaceae or Gramineae family (these are the ones that colloquially known as grasses) are safe, the unknown specimens are better to be avoided.

When feeding hay, look for timothy or meadow hay. Alfalfa or other legumes are not suitable for non-producing animals, they can be given as snacks only.

When you introduce fresh grass at the first time, do it gradually. You can give more and more every day, while check if there's any sign of diarrhea. If there aren't any sympthoms you can turn a cavy from zero to unlimited grass consumption within a week. When grass supply stops for a longer period, you have to start the process again.

Fresh grass must be uncontaminated, that's why do not collect it from areas adjacent to major roads or from places that are used by dogs. In the countryside the task is rather easy, but it is possible even when you are in Dublin and have no own garden. Graveyards are generally dog-free and the grass is excellent.
Note, that the grass in parks and the neighbours' well-maintained lawns are often treated with chemicals.  Those are strictly off-limit!

Unlike fresh grass, hay doesn't contain vitamin C. Cavies cannot synthetise it for themselves. A healthy, non-pregnant or nursing, adult piglet needs approximately 10 mg a day. The right kind of pellets and a handful of fresh grass contains the necessary amount.

No matter if it's fresh or dried grass, you must provide acces all the time, 24 hours a day. Their gastrointestinal tract is long, fibre contains very little nutrition, so they never stop eating. So please do not let them starve.
When a guinea pig doesn't eat, the liver gets damaged. The animal consumes its own fat reserves, but as the level of blood sugar is dropping, ketons are generated. This process is extremely dangerous, without treatment it's often lethal. But even in the best case, damages are permanent. Make sure your guinea pigs always have hay or grass in an accessible place.

When you feed hay or grass, make sure to place them in ground level. Eating from hay racks is very uncomfortable (try to eat in that position) and it can lead to less fibre consumption. 
Unfortunately it causes more waste, since guinea pigs tend not to eat the hay that they peed on and trampled down. This could sound bad at first, but actually we can use this in our advantage. If you place hayboxes in their cage, they could be the place where most urine ends up. Unfortunately the method doesn't work with the poop. The haybox could be anything that has low edges, so that it can keep the hay in, but cavies can enter easily.

Try to buy hay from farmers. It is way cheaper than pet shops and the quality is not worse either. If you don't have a local source, try Hay your way. With an affordable source, you don't even mind if they waste some.