I will be visiting Al Ain shortly and I found reliable information on the oasis, the Al Hili archaeological site as well as the Palace Museum.

However, I found diverging information on the opening/closing times of the Al Jahili Fort. Some sites state that it is open daily except Mondays and other sites state that it is closed on Sundays and Mondays.

Since I was planning to visit on a Sunday, I'd like to have reliable information not to miss out on visiting the fort interior.

Anyone been there recently who can help me out?

We were there in Nov 2013 and I can confirm that the fort IS closed on Mondays. We visited it then and spoke with the Pakistani guard on the gate who let us take a couple of interior photos from it. I guess the main thing we missed was the Thesiger photo exhibition. Apart from a couple of structures in 2 corners it consists mainly of a large courtyard. Perhaps also it is possible to go up into the 4 story tower. If you can't get in then the external views are still worth having.

Unfortunately I don't have personal experience of it on the Sunday - however I DO have the Fort Brochure picked up in the National Museum and it clearly states
"Open Daily except on Mondays
Entrance Free of Charge
Tuesday to Sunday 9.00 am - 5 pm"

The National Museum WAS open on a Sunday (I preferred to spend my late pm there after driving from Dubai via Abu Dhabi as it too is closed on a Monday) and I don't believe that closing on a Sunday is normal/common in the UAE but I cannot of course be 100% certain in this case. Sorry!

For certainty you could try e-mailing the Abu Dhabi Authority for Culture and Heritage which is apparently responsible for the fort
info@cultural.org.ae (according to my brochure)
OR
info@tcaabudhabi.ae (according to the Abu Dhabi tourism and Culture Authority's web site!)
Indeed I see that this latter actually has a contact form for making enquiries!
http://tcaabudhabi.ae/en/about/Pages/contact.us.aspx

The Hili Archaeological Park was open on the Monday by the way - it had a gate and an "office" with a window which might once have been intended for taking money but it was unoccupied and seems to be a fully "free" facility!!.

Thank you very much for your help.

It is very useful as the information I found on the internet mostly tallies with the opening times on your brochure. Since I can only visit on a Sunday or a Monday, I'll opt for a visit on a Sunday. Hopefully I'll be able to visit the fort interior (mainly to take photos of the tower), if not I'll spend more time at the palace museum, oasis and Hili Archaeological Site (possibly stopping at Jebel Hafeet on my way back to Dubai).

I have already filled in the contact form and hopefully I'll be able to get an official reply from them too which I'll then post here too.