Extremely unlikely that the NowTV Gift Cards bought in the UK will work on a NowTV Republic of Ireland account.
Personally i wouldn't risk it myself.
Though you could pop over to live chat for confirmation either way.
See if live chat can give you the definitive answer.
I am based in the UK and not familiar if NowTV ROI offer Gift Cards either on their website or via third party Authorised Republic of Ireland Retailers ?
I had a quick look at Currys website in the Republic of Ireland and couldn't find any NowTV Gift cards only the NowTV sticks bundled with either a two month cinema pass or two month entertainment pass.
Maybe a NowTV ROI customer who uses the Community Forum on here can offer you some advice & help ?