I've often used Sail&Rail tickets to travel between Ireland and the UK. Seat61 tells me that Sail&Rail is also available from the UK to the Isle of Man. However, I cannot find out whether I can use Sail&Rail between Ireland and the Isle of Man. (Tickets from the Republic of Ireland or Northern Ireland would do.)
The Seat61 site has an embedded booking form from Rail Easy, but that just says "No journey found".