Martin is suggesting making fish and chips with fish in batter. Good quality batter often includes some beer to make it lighter: the beer has yeast in it. As Akeisha points out, Martin is Irish, so she/ he is making a joke about adding Guinness to the batter mix. Guinness is a traditional Irish beer - very thick, very strong and quite bitter - probably not the sort of beer that is usually put into fish batter. Although I wouldn't know really - I'm not Irish...

Guinness is a dark Irish dry stout that originated in the brewery of Arthur Guinness at St. James's Gate, Dublin, Ireland, in 1759 . A particularly Irish choice would be to have a drink of Guinness with it. He's Irish.

Guinness is a very popular Irish beer, English men tend to love to eat their meals with beer, for example at a pub

It’s just that Guinness is a famous Irish beer. A very tasty beer, I would add. One of my favourites.