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You have a big Irish family, I'm sure they are boisterous and friendly and not likely to let any guest go un-welcomed at your wedding. Your friends will find someone to party with if they don't get to bring a guest... There is really no reason to do "And Guest", just do a good seating plan so they have folks at their table they have something in common with.

If your intended guest has a significant other, you should be aware of that (If you are not aware, the other is not that significant.) Invite the significant other by naming them on the invitation. Do Not send the significant other a separate invitiation unless you want them to attend the wedding in the event the couple breaks up! Eek!

It's your wedding, so do what reduces your stress. At my wedding, the "no-shows" (i.e. didn't cancel so we had to pay for the dinner they didn't eat) were the "significant others" who I suspect really just didn't want to be at a wedding with a bunch of people they didn't know.

Good luck!!!

Thanks, it helps a bunch. I'd like to hear from some others???

It is proper to invite the adult living at home with his/her parents. If they are over the age of 18 then you would send them a seperate invitation.
You do not have to put plus guest, that way those who know proper etiquette will know that they are not to bring a guest and those that do not know will invite someone anyway.
If there is a couple with small children and you arent having children at the event then you would address it with "Mr. and Mrs. John Doe"....if you are having children then you would address is "Mr. and Mrs. John Doe" and then add the childrens first names as "Jane, Joe, Dolly" underneath their parents names.
If you know someone who is living with someone then it is peoper to invite both of them and to list their names instead of "plus one" and you can use just one invitation.
If the person has roommates you do not have to invite them if you do not want to even though they live in the home with your guest...only if they are dating.
If your guest is exclusivly dating someone, it is proper to find out that persons name and to invite them with your guest....you can just use one invitation.
Always use one invitation when inviting your guests date or plus one even if they arent living together because then they might assume they too can bring a guest and then your head count will double. :-)

Hope this helps. You can google "invitation etiquette" and you will find these guidlines listed on different sites.