Why should you bother? Your EHIC card will get you free emergency cover anywhere in the UK (which I presume includes the IOM). Even without it, I understand that Irish people can access free A&E services in the UK.
That ARB policy is good value. If you're a member of a Trade Union you might be eligible for a discount on the premium (10%-ish). I know there are lots of other similarly-priced annual multi-trip policies, but they usually have much higher excesses and lower benefits in the event of a claim.
The EHIC is valid anywhere in the EU, but it doesn't cover missed flights/cancellations/lost or stolen luggage, etc. That said, if you reckon you won't need that cover, then there's certainly no reason to buy insurance!
Based on the recent necessity to make a claim on travel insurance for a missed departure, a claim which was denied, and keeing in mind the points already mentioned here, I question whether travel insurance is worth the money it costs.
My advice would be to read each policy in full before you purchase. (But who, in reality, does?) It is a safe bet it is full of get out clauses for the insurance company.
Enjoy your holiday anyway.
I usually err on the side of caution .....I too had in mind the 'lost luggage, missed flights, air ambulance' scenarios!
My Credit Union can expect a call from me next week !
Having said that, I had a bad experience with a travel insurance company last year.
Basically I took out a 2-year policy for my trip to Australia/New Zealand/Cook Islands/U.S.,and while to literature offered a "refund if you return home early", the company refused to pay up when I returned home after 11 months.
Five letters to them failed to get my refund.
It was only after I contacted the UK Ombudsman that they finally paid up!
......makes me wonder if they would have paid in the event of a claim ?!!