Unfortunately the early immigration records contain little more than the person's name, age, and country of origin. There were 99 William Brennans who came through Castle Garden (the immigration center that was used prior to the opening of Ellis Island), and 53 who came through Boston in your William's timeframe. Even if you narrow those down to ages close to your William Brennan, there's no other info on the manifest that can be used to verify if any of them are him. Is he in the 1870 census? Have you tried to look for other Brennans in the same town that may have immigrated at the same time?