It's quite an obscure time-period, and my reading has been mostly on a tangential subject, but I'd say it's unlikely. The first bit of evidence are the Pictish depictions (heh) of warriors. The figures are either obviously male, or don't have an obvious gender: there are no obviously female figures (in this one for instance the beardless figures are wearing something over the lower part of their faces). It's not a huge corpus, but I think it's likely that when a Pict pictured (heh) a warrior, they would think of a man.

The other bit of evidence related to possible Pictish woman warriors is the Cáin Adomnáin, Adomnán's law, which was a law passed in 697 at Birr by Adomnán, bishop of Iona (best-known for his biography of Columba), extending the protection given to the Church to women and to "innocent children until they are capable of slaying a man". It specified heavy fines and penance for men who killed women, and also execution in some cases, which was unusual for Irish laws. A century after this law we can expect that there was an expectation that women were not part of warfare. The law was supposed to cover 'Ireland and Britain (in reality the Irish-influenced bits of Scotland).

It's less clear what the situation was before. Was Adomnán extending 'geneva convention'-style protection to a group that hadn't fought before, but had suffered anyway? Or was he, on the other hand, outlawing a practice of woman warriors? The (probably tenth or eleventh century) author of the introduction to the text of the Cáin Adomnáin certainly thought so. The text consists of the introduction, about how women did all the fighting in the old days until Adomnán, at the instigation of his mother, set out to liberate them; a list of the clerics and rulers who guaranteed the law, including 'Bruide mac Derilei, king of the Pict-folk' (interestingly, Der-Ilei was probably his mother); and then the supposed text of the law itself, in proper medieval Irish legal language (so dry: very dry). The text presents the remarkably weird (to modern eyes) picture of a ninth-century guy looking back complacently at all the advancements in the position of women won by seventh-century feminism, but I don't find the historical picture he presents particularly compelling. Even in a more realistic version, I'm not really convinced women fought before this (though, as I said, there's not a huge amount of evidence either way).

Source: T.M Charles-Edwards 'Early Christian Ireland'.