There's no rule to it but I tend to prefer to start of some simple stuff to get warm and the nerves under control. Songs that is tricky for the vocalist should be placed accordingly on the set list. Not too early as the voice should be warm and not too late as he/she might run low on stamina. Also it's a good idea to end the gig with the songs you think will be received best, since people tend to remember that better.
The thing about the vocalist is definately true. With regards to ending on a high, that's a definite, but I have to say that in a pub gig, lots of folks won't remember the las few songs at all
Just remember to leave them on a high so they want to come back, even though they won't remember why they feel good after your gig.
Personally I'd open Set 1 with Whiskey in the Jar because you're in Ireland, it's a great song, and from experience it really works as an opener. If you play the thin Lizzy intro, it lets those who know that version get ready to listed, or if you lurch in Metallica style, it shocks people into listening.
I prefer the Thin Lizzy version (as people hear the guitar then sort them selves out with a seat before the whole band comes in) but with Metallica tone
Let people get drunk before you belt out the Robbie Williams and people WILL sing along (or they're not drunk enough
) I'd do that late in the first set before they get too rowdy(or even move it to the second as you go along if they're not drunk enough). The closer of the first set need to be something that shows you're taking a break, but the audience feels they NEED to come back. U2 Streets would be my choice. Or Sex on Fire, I've seen a fantastic rendition of that once in a pub and everyone waited out till second set.
Sweet Child of mine is a good second set mid player (where it comes on the actual album so to speak) and I would imagine that We are the Champions is a good encore of second set closer in a pub gig, but I've never seen it myself. Mrs Jono used to be a majorette and it was her troupe's exit music with sing-along-effect as people left.
I'm a bit old to know all the songs, but watch for some sour reactions to Lady Gaga and Michael Jackson as they're a bit left field compared to the rest of your set. and take out the kid rock one, it's halloween thus winter. This one won't fit. Play Sweet Home Alabama instead
Overall I'd go for UP down up (set break) up down UP as the general pattern.
That's my take on it. Looking forward to what others say.