Isteach means "in" so tá fáilte romhat isteach is "welcome in", though you might be more likely to say "Please do come in" or such in English, the clear implication is that you have stepped into a specific room or building, rather than a general welcome on arrival at a place.
In DL Irish, there is always a blue "sound" icon at the top of the discussion page, which quite freqently (one out of three to four?) gives a sound sample (as is on this page as well).
Now the quality of the speaker is considered extremely poor by the experts, but I still appreciate a poor indication more than no indication at all. From where I come, "romhat" would be pronounced (and silently read) as the english "rum hat", which definitely is much worse, than the poor indication.
I'm aware that this is not common modern English usage but I would read this (and other "singular-you are" constructs) with a "thou art". Quaker. Plain speech. All that. Plus it distinguishes singular from plural, especially if one ignores the bleedin' silly T-V distinction that we got mostly from the French. Unfortunately DL does not appear to approve.