Isle of Man is not in UK or EU
The Isle of Man is a "Crown dependency" of Great Britain
Is not legally technically part of the United Kingdom or the European Union.
It has islanders democratically elected as "Members of the House of Keys" to the island parliament called Tynwald claiming to be 1000 years old. This parliament resides at the House of Keys in Douglas and has a publicly observed summer open air meeting. For all practical purposes, as short term non-working visitors to this island, it is like visiting the UK.
They have their own Police force.
It is in effect similar in principle to Jersey and Guernsey islands which each have their own parliaments.
For our purposes
Visiting the IOM, and not working, makes it the same as visiting the UK.
UK border controls take effect as you fly internationally into UK or ferry into the UK.
Then you need identification to board a short distance plane from UK to IOM. Similarly you need identification to board a ferry from England, either Liverpool or Heysham in Lancashire, to IOM.
Note: I am unable to comment about the Dublin or Belfast ferry summer routes. I guess it is the same in principle.
About the Isle of Man
Regular news about the Isle of Man under the Manx title of Ellen Vannin
Receives UK sourced television broadcasts of the Granada North-West of England region including news from BBC, ITV, Channel 4 and Channel 5.
There are two Standard Definition multiplexes and one High Definition multiplex covering BBC 1, BBC 2, BBC 3 and Channel 4.
Receives UK sourced radio broadcasts of the BBC.
There are also the Isle of Man own commercial radio services called 3FM and Energy FM.