The passageway is long and narrow. There is one point where the stones have shifted a little where there might be a squeeze. You can ask the guide if your husband can enter last so that if he can't go through, he can go back out without disrupting the others on the tour. I thought the interior was pretty spacious but they do pack a lot of people into it. That made me feel claustrophobic. Less people inside would have been better.

The visitor's center has a mock up of the interior at winter solstice. So if your husband has problems entering Newgrange he can view that with no problem.