I am travelling on vacation to the UK and Ireland in August 2013.

I am travelling on vacation to the UK and Ireland in August 2013. I wish to purchase a SIM card to place in my unlocked Samsung Galaxy Ace handset. SIM cards from Tesco or similar are only good in UK and Northern Ireland. In southern Ireland I presume it will go into roaming. Will this incur further costs, when mostly I will be using data for accommodation or /sightseeing purposes??? Any suggestions of a suitable SIM card????

Yes a UK SIM card will convert to International Roaming rates once in the Republic of Ireland. The roaming rates for Ireland and Western Europe aren't usually too high, so if you are only using your phone very infrequently then it probably isn't worth buying a second SIM card for Ireland. However if you plan on using your phone more frequently, especially for data usage, it's probably worth your while investing in an Irish SIM card too.

Alternatively, purchase a UK Vodafone SIM on a 30 day rolling contract, and then purchase an 'Ireland Plus' add-on (it costs about 5 or 10GBP on top of the monthly fee). This allows call, text and data usage included in the contract to be used in Ireland as well as the UK. Vodafone rates also include data usage on BT wi-fi hotspots in the UK.

I don't know if such add-ons are available on pay-as-you-go tariffs or on other networks, but this is probably worth looking into.

Thanks for your advice.  I did some homework on the Vodafone site and see that it is only good for Vodafone customers on monthly contracts, with a Northern Ireland postcode.  I will keep searching and am currently finding that a basic Tesco, O2 pay & Go and an 0044.co.uk plan may do what we want to do.  Thanks.