>> Tipperary, some in Kildare and also Meath

This is an odd mix of counties, but as they appear in quantity in Leitrim and Fermanagh (Sligo too I think - actually they're all over the place!) They are most likely just a fashion or trend that gravestones went through, just as tacky Celtic crosses were in voguea few decades ago.

The ones in the article are earlier than most of the ones I know of though, so the idea behind them could be different. Saying that here is one that could link them up because it has the skull (minus bones) and a timer. The carvings seem very similar in execution : http://www.megalithomania.com/show/site/900/2667 . The words 'Memento Mori' presumably mean 'Remember Death'. It is thought that this stone was perhaps based on the drawings of a child because of the face.