Janeom,
If you are interested in the house you should get a surveyors report done - usual cost around €300 I think (not sure what the usual cost is in Belfast). It may seem like a lot to pay out particularly if you end up not buying the house but the report should show the level of insulation in the house and might show up why it has been on the market for over a year.
With regard to putting in an offer; do you know what equivalent size houses are selling for in the area? Do you now has the price been the same for the full year the house has been on the market or has it dropped over the 12 months?
Without knowing the house, area or history I would put in an initial offer of £150,000 stg and see what response you get - it doesn't cost you anything to put in an offer and it makes no difference to you if the seller doesn't accept you can put in a higher offer.
In my opinion it might be better to wait until after a surveyors report before putting in a bid as the report could possibly give you ammunition to support your lower bid. Also, don't be surprised if "another bidder" suddenly and surprisingly emerges out of nowhere just as you put in your bid - it has been reported on this forum that this tactic may used by estate agents in the south from time to time to try and get higher bids out of people.
Let us know how you get on
efm