Another Celtic queen Tacitus wrote about was Brigantes Queen Cartimandua, a contemporary of Boudicca's, who was apparently more level-headed than the Iceni queen, for she did not join the rebellion against Rome in 61 AD, but remained loyal to Rome throughout her reign.
Cartimandua came into power under Claudius around the time of the Roman conquest in 43 AD. She proved her loyalty to Claudius by turning the rebel leader Caratcacus over to the Romans when he came to her for aid in 51 AD. Having rewarded the Roman emperor with the greatest trophy of his triumph, she was in turn rewarded with great wealth.
What little we know of Cartimandua was written by the Roman historian Tacitus, who describes her as "illustrious of birth." She does appear to have inherited her power and ruled in her own right. Though married to Venutius, who was king and warlord of the Brigantes, Tacitus confirms that she ruled the roost as the living symbol of the goddess Brigantia. She eventually divorced Venutius to marry his charioteer (or arms bearer), Vellocatus.
Tacitus recorded that their divorce caused civil war among the Brigantes, largely because Venutius was favoured among their people. Nonetheless, preferring to have her illegitimate husband by her side, Cartimandua allowed her libido and ferocious temper to prevail. (Tacitus 3.45). In 57, Venutius gathered support among the anti-Roman faction of the Brigantes and retaliated. Rome intervened and saved Cartimandua, but lost Brigantia to the hostile king. Cartimandua's actions gave the Romans the excuse they needed to seize Brigantia and restore her power.
Years later, during the year of the four emperors in 69 AD, Venutius staged another revolt. Rome was only able to send auxiliaries this time to help Cartimandua escape, leaving Venutius in charge, at which point the Brigantes queen faded from history.
Note on Tacitus: What makes Tacitus of particular interest to Romano-Celtic history of the 1st century AD is that he had an eye-witness in Britain at the time of Cartimandua and Boudicca. His father-in-law, Gnaeus Julius Agricola, was stationed there at the time and served under Gaius Suetonius Paulinus, the Roman general who defeated Boudicca.
Sources:
Wikipedia
The Heroic Age, Issue 1, Spring/Summer 1999."Brigantia, Cartimandua and Gwenhwyfar"