Thursday night victory at the Aviva Stadium completed a Nations League double for the Republic against Finland after the 2-1 win in Helsinki last month.
But it was the Finns who made the better start and Kann Kairinen fired off-traget before seeing his shot blocked.
The Irish saw less of the ball but they carried a threat throughtout and the home crowd celebrated when Ferguson played in Sammie Szmodics and the Ipswich forward rounded keeper Lukas Hradecky before slotting home.
However, the flag was raised for offside but the hosts were soon on the attack again with Ferguson's deflected long-range strike fizzing over.
Lod went so close to putting Finland in front when he curled against the post before Benjamin Hallman headed just wide.
Back came the Republic with an unmarked Matt Doherty running in at the backpost to meet a corner but he headed wide.
The same post came to Ireland's rescue again as Oliver Antman cut inside and his shot was goalbound until Nathan Collins got a vital touch to divert it on to the woodwork.
It proved to be a crucial intervention with the deadlock broken in the final minute of the first half as Johnston jinked his way to the byline and his dinked cross found Ferguson, who leaped highest to nod in from a yard out.
Image source, Inpho
