I will go along with a combination of JonW and Fergie.

Kavanagh knows the woman is not good for him but he throws caution to the wind and starts a relatonship anyway. I think perhaps she may have been a bit iof a hooker. The queens of Hearts I think refers to Dublin city and the tarts are the hookers in that area ?. The secret sign may have been some attempt by Kavanagh to educate this woman about some of the finer things in life. Later after the affair has ended he still sees her walking the streets. I guess only Kavanagh knows for sure, but it is an interesting exercise trying to read his mind.