Hi Michael,
Thanks for taking the time to ask your question!
As you say, most tour buses are standard coach-type buses and have no amendments for wheelchair access. While people with disabilities do use the tours, they either need to be able to get on/off themselves or have their own assistance with them.
The
Irish Wheelchair Association may be able to help with information and
Transport for Ireland has some public transport information which may also be an option for independent travel while on the island of Ireland. A quick internet search has found the following tour companies,
Disabled Holidays, Limitless Travel, and
Lynott Tours, while we have no direct dealings with them they may be able to help with trip arrangements.
Regarding the Special Needs Group you mentioned, it would probably be best to contact them directly as they are based in the USA and we have no further information on their services.
We hope this helps,
Martin