You may say 'goshoukai arigatou gozaimasu.'
Gracias por la invitación.
If you mean a job offer, you begin for thanking them for their kind consideration. Then you must accept or decline. You are not REQUIRED to give a reason, but if you ever expect to go back that way...
SAY iam praying from God keep you for me, forever.
Go raibh míle maith agat as ucht an cuireadh (singular) / Go raibh míle maith agaibh as ucht an cuireadh (plural)
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