Vitae or "Lives" of Saint Brigit: Great Places to Learn Lots About Her
Although there is some debate—to my mind, now settled—about which is the earliest “Life” or “Vita” of Saint Brigit, there is no doubt that Cogitosus was a monk at Kildare around two centuries after Brigit's birth.
His life has its miracles, certainly, but contains far fewer pagan elements than in later vitae. See Lisa Bitel's account of why that might be in “St. Brigit of Ireland: From VirginSaint to Fertility Goddess”.
Meanwhile, read his Vita Sanctae Brigidae itself*, translated by Liam de Paor in this book:
Another translation, with excellent commentary, can be found through JSTOR (if you have access) or your library: “Cogitosus's ‘Life of St Brigit’ Content and Value” by Sean Connolly and J.-M. Picard. The Journal of the Royal Society of Antiquaries of Ireland, Vol. 117 (1987), pp. 5-27
What is likely the third oldest vita of Saint Brigit is Bethu Brigte or "The Irish Life of Saint Brigit", called so because it was written not in Latin, but in Irish. Its English translation can be found online at CELT, from University College Cork.
The second oldest vita, once thought the oldest and therefore called Vita Prima Sanctae Brigitae, is not to my knowledge available online. It can be found through JSTOR, if you have access to it, or through your library (possibly interlibrary loan or article request), in the article “Vita Prima Sanctae Brigitae Background and Historical Value” by Seán Connolly. The Journal of the Royal Society of Antiquaries of Ireland, Vol. 119 (1989), pp. 5-49.
For more links to sources on Saint Brigit visit her page on Monachos.net.
* You will have to rotate the view to do so.