I haven't been to Tara but I have been to Newgrange a number of times. For historic reasons alone I think it is worth a visit.
First you go to the visitor centre where there are exhibits explaining life in the Boyne valley through the ages. Of course there's explanations of the Newgrange site itself, details of how the mound was constructed and the possible uses it was put to over the centuries. There's also an audio visual show in several European languages.
To visit the Newgrange site itself, you must make a short walk through the grounds and across the Boyne to board a bus which will take you there and back. You can't go straight to the site yourself. You can buy tickets at the counter in the visitor centre giving you an alloted time to depart.
There are a number of combinations of trips available, including the other sites at Knowth and Dowth. If you're travelling early in the year, there shouldn't be that much of a wait but even so, my advice is to go to the centre as early as possible in the day, especially if at the weekend, and book your bus trip as soon as you arrive. You never know, there could be a large group ahead of you on the day.
Numbers entering Newgrange itself is strictly limited and always accompanied by a helpful and informative guide. Going into the passageway isn't recommended for the claustrophobic or generously proportioned. The interior is surprisingly small. When everyone is inside the sunrise on the shortest day is simulated.
Back at the centre, there is a functional restaurant serving light meals, sandwiches, etc and there are some excellent views across the valley.
I reckon a visit will take approx 1.5 to 2 hours.
I hope you enjoy your visit and that if you go to Tara, you'll share your impressions with us. Travel safely.