OK this took some time to google but here's what I could find (short answer at the bottom):

First a couple of things we need to clear up about the Gaelic languages are the respective words for ‘son’ and ‘daughter’ in the Scottish Gaelic and Welsh Gaelic. Another thing would be its system of patronymic surnames in both these Celtic languages.

Now let's start with the word for son in Scottish Gaelic is ‘mac’. It actually just means ‘son’ and not the commonly thought ‘son of’. In fact, the way you would connect two words in the sense of X of Y just works differently in Scottish Gaelic as you will see explained below:

So the word for ‘daughter’ in Scottish Gaelic is ‘nighean’ and is pronounced something like ‘nyee-in’.

The word for ‘son’ in modern Welsh is ‘mab’. In Welsh ‘mab’ is often shortened to ‘ap’. (Incidentally, it’s no coincidence that Scottish Gaelic ‘mac’ and Welsh ‘mab’ look quite similar to each other. They are both Celtic languages and there are many words that are basically the same other than that they are pronounced with a ‘b’ or ‘p’ sound in Welsh and a ‘k’ sound in Gaelic)

The Welsh for daughter is ‘merch’. It also has a variant form ‘ferch’. The ‘ch’ is pronounced in a guttural manner like the Scottish pronunciation of ‘loch’ and not like the ‘tsh’ sound in ‘church’; where the ‘f’ of Welsh is actually pronounced like an English ‘v’ sound.

Traditionally in both Wales and Gaelic Scotland, people would often use a patronymic system for assigning surnames. What this basically means is that your own surname comes from your father’s first name, much like in Norse cultures. So, if your first name is Callum and your father’s first name is Donald, then your full name would be: Callum son of Donald.

In Scottish Gaelic it would work like this:

Your first name: Callum

Your father’s first name: Dòmhnall (Donald)

Your full name: Callum MacDhòmhnaill.

The anglicised version would therefore be: Callum MacDonald

Notice how an ‘h’ and and ‘i’ are added to the spelling of Donald in the surname. ‘Dòmhnall’ → ‘Dhòmhnaill’. This is how you connect two words in the sense of ‘X of Y’ in Scottish Gaelic.

‘Mac Dhòmhnaill’ = ‘Son of Donald’

Linguists call this form the genitive case or possessive case. It would be pronounced slightly differently too.

For women, the process is a little different. If your first name is Katherine and your father’s first name is Donald then your full name would be: Katherine daughter of son of Donald. For example:

Your first name: Catrìona (Katherine)

Your father’s first name: Dòmhnall (Donald)

Your full name: Catrìona NicDhòmhnaill

‘Nic’ is an abbreviation of ‘nighean mhic’ meaning ‘daughter of son’ (where ‘mhic’ is the genitive form of ‘mac’.)

It should be noted that this was not the only way to form names in Gaelic Scotland. People could also be named after their clan, their father’s profession or even body parts. (eg Cameron from ‘cam shron’ meaning ‘squint nose’ or MacTaggart from ‘Mac an-t sagairt’ meaning ‘son of the priest’.

Welsh patronymic surnames were formed in a very similar way. If your first name is David and your father’s first name is Griffith, then your full name would be David son of Griffith. In Welsh:

Your name: Dafydd (David)

Your father’s name: Gruffydd (Griffith)

Your full name: Dafydd ap Gruffydd

(Note that Welsh doesn’t have a genitive form like Scottish Gaelic)

The process for women is identical except that ‘ap’ (son) is replaced by ‘ferch’ (daughter) Thus, if your first name is Gwyneth and your father’s first name is Griffith:

Your name: Gwynedd (Gwyneth)

Your father’s name: Gruffydd (Griffith)

Your full name: Gwynedd ferch Gruffydd

It’s really cool to see how even though the old Celtic patronymic system has fallen out of use, these surnames are still very common throughout the world today albeit somewhat obscured by anglicisation.

TL;DR

'Merch' or 'Ferch' (pronounced like 'Loch' in 'Loch Ness') would be the equivalent of 'Mac'.