Rubuta ci gaba ga wannan sakin layi - An sanar da Brennan a matsayin daya daga cikin sabbin ma'aikata zuwa makarantar Nigel Carolan ta Connacht kafin kakar 2015/16. Matsar zuwa makarantar Connacht ta kuma gan shi ya koma Galwegians a matakin manya, bayan ya buga wasan ƙwallon ƙafa na matasa ga ƙungiyar. A lokacinsa na farko a makarantar kimiyya, Brennan ya fito a kai a kai ga Galwegians a cikin League na All-Ireland kuma ya buga wa Connacht Eagles, na biyu na lardin. Ya fara horo tare da ƙungiyar farko kuma an kira shi a cikin babban rukuni a karon farko don 2015 <unk> 16 Pro12
wasan da Ulster a ranar 1 ga Afrilu 2016, kodayake bai fito daga benci ba. Brennan na ɗaya daga cikin 'yan wasa huɗu na makarantar da suka horar da Connacht a lokacin kakar wasa ba tare da samun damar shiga babbar ƙungiyar ba. Lokacin da ƙungiyar ta cancanci shiga wasan karshe na Pro12, duk da haka, sauran ƙungiyar sun biya don kawo su huɗu zuwa wasan a Edinburgh don girmama gudummawar da suka bayar.