Possible Clues:

• Currently, not on Eire letters
• ___ law (1840's discovery)
• There'd be some resistence to having this on Irish letters
• There'd be some resistence to putting this on an envelope in Ireland
• Some resistance shown on Royal Mail?
• In Eire there'd be resistance to having this on Government letters
• There would be some resistance to having this on official envelopes in Ireland
• 'Oh, that's the writing of the Resistance (4)'
• Electric resistance measures
• __ Law