The past tense of "is" is "ba", so it would be "Ba leis an smaoineamh", but in this case I think I would probably use the emphatic form: "Ba leisean an smaoineamh". "Is" only has two tenses: "is" is the present/future tense and "ba" is the past/conditional tense. "Ba" and all its various forms (interrogative "ar(bh)", negative "níor(bh)", etc.) are followed by séimhiú (lenition).