There are 4 main groups of texts that form the known basis for Celtic mythology, called cycles. These are the Invasions Cycle, Ulster Cycle, Fenian Cycle, and Historical Cycle.

The Invasions Cycle is ostensibly about the Celts repelling invaders. Historically, there were multiple attempted invasions of Ireland, and these tales portray those accounts in a mythic light. These tales focus on the people known as Tuatha de Danaan.

The Ulster cycle focuses on the hero Cuchulainn, the greatest hero of Celtic mythology. It follows his life, beginning with how he won his name at the age of 6, to his training with Scathach, to his exploits in battle, and finally to his death.

The Fenian cycle follows Finn mac Cumaill and the Fianna, his soldiers. This cycle is from a later epoch, and portrays the warriors as not just bloodthirsty, but also skilled in tasks like chess.

The Historical cycle isn't one I know very well, but it allegedly has to do with a later series of Celtic kings.

The cycles tend to focus on a few key concepts, notably the power of a warrior's spoken word. For example, in the Invasions cycle, one of the best stories* is about the children of Tiurenn, who offended Lugh of the Long Arm by killing his father (it was a blood feud). In recompense, they agreed to get a few items for Lugh, such as a few apples and a kettle. Lugh then decides that the apples come from the garden of the Hesperides (a la Hercules) and the kettle is at the bottom of the sea, guarded by mermaids. The brothers are bound by their word to complete their tasks, and their heroic efforts to do so gain their honor.

*best is subjective

I definitely recommend trying to read some of the Celtic myths, my favorite book for doing so is Myths and Legends by William Doty and Jake Jackson.

Asking for clarification- do you mean which myths and sagas were influential on Celtic culture, or are you just asking to learn what Celtic myths and sagas existed?