I tend to feel bad about how terrible I am at making estimates, but the Irish news websites today make me feel better. The Irish Times guesses that thirty-five thousand people marched in today's Rally for Life in Dublin, while The Irish Independent hikes that up to fifty thousand.
Somehow, I managed to not realize there was a rally on today. (I have just come back from a week on the continent, in my defence.) I was taking a bus into the city centre and didn't know why the traffic had been stopped. I had some business to do in O'Connell Street, and was there for about twenty minutes. In all that time, the flood of demonstrators heading to Kildare Street never slackened. After I had followed them there, someone announced from the stage that the march was still coming down O'Connell Street.
Bad at estimates as I am, I think it was a lot more than thirty-five thousand.