I have always understood the similarity between italian,French, Spanish and Portuguese but ancient Irish and welsh are strikingly similar to Latin tongues almost as if there was ancient link.....a pre-Gallic Celtic link.
Like most languages, Irish can be divided into 3 periods: Old Irish, Middle Irish, and Modern Irish, where the earliest of these was used from about 600 CE.
But with Irish, there is one documented earlier period called Ogham Irish after the Ogham alphabet which was used to record it on standing stones throughout Ireland (mostly in the south) and the Irish colonies in Wales. These stones date from around the start of the Common Era to the 500s.
Ogham Irish is so different from later forms, that it is considered one of the tipping points when new peoples or cultures may have entered Ireland. As adamo has said, it looks a lot like Latin especially with vowels for word endings.
This difference between Ogham and later forms of Irish have led some people to claim that it is a surviving remnant of a much earlier language which became just a sacred language in the CE, much like Sanskrit and Church Latin.
But the counter argument to this is Ptolemy's maps which use the same form of Irish. Since it is believed that Ptolemy got his information, indirectly, from common seamen who would have spoken the current language and not some sacred one, this indicates that the Ogham stones also used the common form of Irish spoken at that time.
Below are some examples from J. P. Mallory's book "The Origins of the Irish" and next to that is the Ogham language.
In Old and Modern Irish, aid(h) is pronounced like 'ah' or 'ee', and th is just pronounce as 'h'.
It is believed that the name for the ancient capital of Ulster, Eamhain Macha derived it's name from the Isamnion on Ptolemy's map where by syncopation we get I(s)AM(ni)ON to IAMON, or Eamhain. Macha was the goddess of the Ulaid (Uly).

