Some household insurers will do it as an add-on to the house. Though your best bet is to contact a broker, as they would have access to other companies who do it. Eagle Star do caravan insurance, though i think Hickey Clarke & Langan minimum premium is lower than theirs. (they are a broker with a scheme for caravans). Also a company called Finsure/Alan B Kidd do it (again a broker with a scheme).
I agree with peteb. Hickey, Clarke and Langan Insurances in Donegal offer a mobile home policy that is underwritten by a Lloyds scheme. Rates are very competitive but cover may be subject to some restrictions. contact them at (074) 912 6688 or mailto: info@hcl.ie.