Answer by taking a quote from the following article:

The Illyrians (Ancient Greek: Illurioi, Illyrioi; Latin: Illyrii or Illyri) were a group of Indo-European tribes in antiquity, who inhabited part of the western Balkans.  The territory the Illyrians inhabited came to be known as Illyria to Greek and Roman authors, who identified a territory that corresponds to Croatia, Bosnia and Herzegovina, Slovenia, Montenegro, part of Serbia and most of central and northern Albania, between the Adriatic Sea in the west, the Drava river in the north, the Morava river in the east and the mouth of the Aoos river in the south. The first account of Illyrian peoples comes from the Periplus of Pseudo-Scylax, an ancient Greek text of the middle of the 4th century BC that describes coastal passages in the Mediterranean. The name "Illyrians", as applied by the ancient Greeks to their northern neighbors, may have referred to a broad, ill-defined group of peoples, and it is today unclear to what extent they were linguistically and culturally homogeneous.

Illyria appears in Greco-Roman historiography from the 4th century BC. The Illyrians formed several kingdoms in the central Balkans, and the first known Illyrian king was Bardyllis. Illyrian kingdoms were often at war with ancient Macedonia, and the Illyrian pirates were also a significant danger to neighbouring peoples. At the Neretva Delta, there was a strong Hellenistic influence on the Illyrian tribe of Daors. Their capital was Daorson located in Osanici near Stolac in Herzegovina, which became the main center of classical Illyrian culture. Daorson, during the 4th century BC, was surrounded by megalithic, 5 meter high stonewalls, composed out of large trapeze stones blocks. Daors also made unique bronze coins and sculptures. The Illyrians even conquered Greek colonies on the Dalmatian islands. Queen Teuta was famous for having waged wars against the Romans.  After Philip II of Macedon defeated Bardylis (358 BC), the Grabaei under Grabos became the strongest state in Illyria. Philip II killed 7,000 Illyrians in a great victory and annexed the territory up to Lake Ohrid. Next, Philip II reduced the Grabaei, and then went for the Ardiaei, defeated the Triballi (339 BC), and fought with Pleurias (337 BC).  In the Illyrian Wars of 229 BC, 219 BC and 168 BC Rome overran the Illyrian settlements and suppressed the piracy that had made the Adriatic unsafe for Italian commerce. There were three campaigns, the first against Teuta the second against Demetrius of Pharos and the third against Gentius. The initial campaign in 229 BC marks the first time that the Roman Navy crossed the Adriatic Sea to launch an invasion.  The Roman Republic subdued the Illyrians during the 2nd century BC. An Illyrian revolt was crushed under Augustus, resulting in the division of Illyria in the provinces of Pannonia in the north and Dalmatia in the south.

Were they peaceful or warlike?
After Philip II of Macedon defeated Bardylis (358 BC), the Grabaei under Grabos became the strongest state in Illyria.