Adrahil I was the first Prince of Dor-en-Ernil. When King Ondoher led his army to face the Wainriders who threatened Gondor, Adrahil commanded the army's left wing. The soldiers of the right wing were under the command of the King's nephew Minohtar, while the centre was commanded by the king himself and his heir Artamir.
Ondoher's enterprise came to a devastating end: he was surprised by the Wainriders, and his forces were overwhelmed. Adrahil's soldiers, however, were shielded from the attack and survived it. Ondoher and Artamir had both been killed, and Minohtar took charge, sending Adrahil south into Ithilien to form a defensive line from Cair Andros to the Ephel Dúath.
Adrahil's story was never completed, so his fate is uncertain. We know that the Wainriders eventually broke through Minohtar's more northerly defence, but whether they penetrated to Adrahil's position is unknown. If he survived the initial Wainrider assault, it seems likely that Adrahil would have joined with Eärnil in his counter-attack, seeing the ultimate defeat of the Wainriders at the Battle of the Camp.
Adrahil's place in history is difficult to define: he seems to have been an ancestor of Prince Imrahil of Dol Amroth (especially since he shares his name with Imrahil's father). If he belonged to this line, though, he lived more than a century before Galador, who is specifically named as the first Prince of Dol Amroth. The dating is consistent with Adrahil being the father of Imrazôr, and hence Galador's grandfather, but the evidence on this point is slim.