Hi Rui
I am sorry no-one has replied. There are certainly wargamers in Northern Ireland but it seems they do not use this forum.
As I am a senior citizen, solo wargamer and I buy most of my supplies online I am not your best source but I will try.
I cannot speak for Dungannon but am aware of two established model shops in central Belfast.
www.modellersnook.co.uk/ www.myoldtoybox.co.uk/ They are close together and to the GW store in CastleCourt.
As to organised gaming, you could start with the University group
https://www.dragonslayers.org.uk/ I think they are fantasy etc but could be a source of contacts.
You should also get in touch with the excellent Wee Gamers:
https://en-gb.facebook.com/weegamers/ (I do not use Facebook myself but what can you do?)
If you are here in June, I have seen historical wargames played at Q-Con
www.q-con.org.uk/ I hope some of this is useful and I welcome you into our land.
Peter